Substitution variables in Oracle are used to replace a value dynamically during the execution of SQL statements. The behavior of these variables is well-documented:
* C. A substitution variable prefixed with & always prompts only once for a value in a session: This is true. In a session, when you use a single ampersand (&), SQL*Plus or SQL Developer will prompt for the value the first time the variable is encountered. The value for this variable will then be reused for the remainder of the session unless it is redefined.
* D. A substitution variable can be used with any clause in a SELECT statement: Substitution variables can be placed in any part of a SQL statement, including the SELECT, WHERE, GROUP BY, ORDER BY, etc. They are not limited to any specific clause.
References:
* Oracle SQL*Plus User's Guide and Reference, which discusses substitution variables.

Key components of an Entity-Relationship Model (ERM) include:
* B. an attribute: Attributes are properties or characteristics of an entity, such as a person's name, date of birth, etc., and are essential in describing the data aspects of an entity in an ER model.
* E. a relationship: Relationships describe how entities interact with each other within the database structure, such as a customer placing an order.
* F. an entity: Entities are the key components of an ER model, representing objects or things within the domain that have a distinct existence, like 'Customer' or 'Order'.
Incorrect options:
* A: A table is a database structure used to implement an entity in relational databases, not a component of the ER model itself.
* C: While unique identifiers are crucial in database implementation (typically as primary keys), they are a specific attribute type, not a general component of ER models.
* D: An activity is not a component of an ER model; activities relate more to process models or behavioral models in systems design.