An Access Control Object (ACO) is a CDS annotation that defines the access control rules for a CDS view entity. An ACO consists of one or more clauses that specify the role name, the data source, the access conditions, and the return code of the authority check12. Some of the clauses that are used in an ACO are:
* Where (to specify the access conditions): This clause is used to define the logical expression that determines whether a user has access to the data source or not. The expression can use the fields of the data source, the parameters of the CDS view entity, or the predefined variables $user and $session. The expression can also use the functions check_authorization and check_role to perform additional authority checks12.
* Define role (to specify the role name): This clause is used to assign a name to the role that is defined by the ACO. The role name must be unique within the namespace of the CDS view entity and must not contain any special characters. The role name can be used to reference the ACO in other annotations, such as @AccessControl.authorizationCheck or @AccessControl.grant12.
* Revoke (to remove access to the data source): This clause is used to explicitly deny access to the data source for a user who meets the conditions of the where clause. The revoke clause overrides any grant clause that might grant access to the same user. The revoke clause can be used to implement the principle of least privilege or to enforce data segregation12.
You cannot do any of the following:
* Grant (to identify the data source): This is not a valid clause in an ACO. The grant clause is a separate annotation that is used to grant access to a CDS view entity or a data source for a user who has a specific role. The grant clause can reference an ACO by its role name to apply the access conditions defined by the ACO12.
* Return code (to assign the return code of the authority check): This is not a valid clause in an ACO. The return code of the authority check is a predefined variable that is set by the system after performing the access control check. The return code can be used in the where clause of the ACO to specify different access conditions based on the outcome of the check12.
References: 1: Access Control Objects - ABAP Keyword Documentation - SAP Online Help 2: Access Control in Core Data Services (CDS) | SAP Help Portal

Based on the given information, the spfli database table should have the following general settings:
"Storage Type" to "Row Store": This setting determines how the data is stored in the SAP HANA database. Row store is suitable for tables that are accessed by primary key or by a small number of columns. Column store is suitable for tables that are accessed by a large number of columns or by complex analytical queries. Since the spfli table is a large table with over one million rows, and this program is the only one in the system that accesses the table, it is likely that the program will use primary key access or simple queries to access the table. Therefore, row store is a better choice than column store for this table12.
"Load Unit" to "Page Loadable": This setting determines how the data is loaded into the memory when the table is accessed. Page loadable means that the data is loaded in pages of 16 KB each, and only the pages that are needed are loaded. Column loadable means that the data is loaded in columns, and only the columns that are needed are loaded. Since the spfli table is a row store table, and this program will run rarely, it is more efficient to use page loadable than column loadable for this table. Page loadable will reduce the memory consumption and the loading time of the table13.

Explanation
To adhere to the most recent ABAP SQL syntax conventions from SAP, you must insert the "INTO TABLE
@gt flights" clause on line #4 to complete the SQL statement. This is because the INTO or APPENDING clause should be specified immediately after the SELECT clause, according to the ABAP SQL syntax conventions1. The INTO or APPENDING clause defines the data object to which the results set of the SELECT statement is assigned. The data object can be an internal table, a work area, or an inline declaration.
In this case, the data object is an internal table named gt_flights, which is created using the inline declaration operator @DATA. The inline declaration operator allows you to declare and create a data object in the same statement where it is used, without the need for a separate DATA statement2.
The other lines are not suitable for inserting the "INTO TABLE @gt flights" clause, as they would violate the ABAP SQL syntax conventions or cause syntax errors. These lines are:
#6: This line is not suitable for inserting the "INTO TABLE @gt flights" clause, as it would cause a syntax error. This is because the FROM clause must be specified before the INTO or APPENDING clause, according to the ABAP SQL syntax conventions1. The FROM clause defines the data sources from which the data is read, such as database tables, CDS view entities, or CDS DDIC-based views. In this case, the data source is the database table flights.
#8: This line is not suitable for inserting the "INTO TABLE @gt flights" clause, as it would cause a syntax error. This is because the ORDER BY clause must be specified after the INTO or APPENDING clause, according to the ABAP SQL syntax conventions1. The ORDER BY clause defines the sort order of the results set of the SELECT statement. In this case, the results set is sorted by the fields carrid, connid, and fltime.
#15: This line is not suitable for inserting the "INTO TABLE @gt flights" clause, as it would violate the ABAP SQL syntax conventions. This is because the INTO or APPENDING clause should be specified as close as possible to the SELECT clause, according to the ABAP SQL syntax conventions1. The INTO or APPENDING clause should not be separated from the SELECT clause by other clauses, such as the WHERE clause, the GROUP BY clause, the HAVING clause, the UNION clause, or the ORDER BY clause. This is to improve the readability and maintainability of the ABAP SQL statement.
References: SELECT - ABAP Keyword Documentation, Inline Declarations - ABAP Keyword Documentation

Explanation
The statement that can be used to change the contents of a row of data in an internal table is MODIFY table.
The MODIFY table statement can be used to change the contents of one or more rows of an internal table, either by specifying the table index, the table key, or a condition. The MODIFY table statement can also be used to change the contents of a database table, by specifying the table name and a work area or an internal table. The MODIFY table statement can use the TRANSPORTING addition to specify which fields should be changed, and the WHERE addition to specify which rows should be changed.
The other statements are not suitable for changing the contents of a row of data in an internal table, as they have different purposes and effects. These statements are:
APPEND table: This statement can be used to add a new row of data to the end of an internal table, either by specifying a work area or an inline declaration. The APPEND table statement does not change the existing rows of the internal table, but only increases the number of rows by one.
INSERT table: This statement can be used to insert a new row of data into an internal table, either by specifying the table index, the table key, or a sorted position. The INSERT table statement does not change the existing rows of the internal table, but only shifts them to make room for the new row. The INSERT table statement can also be used to insert a new row of data into a database table, by specifying the table name and a work area or an inline declaration.
UPDATE table: This statement can be used to update the contents of a database table, by specifying the table name and a work area or an internal table. The UPDATE table statement can use the SET addition to specify which fields should be updated, and the WHERE addition to specify which rows should be updated. The UPDATE table statement does not affect the internal table, but only the corresponding database table.
References: MODIFY table - ABAP Keyword Documentation, APPEND table - ABAP Keyword Documentation, INSERT table - ABAP Keyword Documentation, UPDATE table - ABAP Keyword Documentation