The value of a macro variable is resolved at the time the DATA step is compiled. When the 'data test' step is executed, the value of '&my_var' is 10 because the macro variable was defined before the DATA step. Changing the value of the macro variable after the DATA step has been compiled does not affect the value assigned to 'x'.

This approach first creates a new dataset 'AgeGroups' where customers are categorized based on their age using IF-THEN statements. Then, PROC MEANS is applied to this new dataset with 'AgeGroup' in the CLASS statement to calculate the average purchase amount for each age group. Option A uses 'Age' directly as a CLASS variable, which would not create the desired age groups. Option B does not categorize customers into age groups. Option D uses WHERE statements to filter data but does not categorize customers into age groups. Option E uses incorrect syntax for specifying ranges in the WHERE statement.

The SAS program provided includes a PROC FORMAT step that defines a numeric format called Syn and applies this format to the variable answer in the PROC PRINT step. The format maps the value '0' to 'No', '1' to 'Yes', and all other values to 'Unknown'.
However, there is a syntax error in the PROC FORMAT step: it uses a dollar sign before Syn, which is used to indicate character formats, but the format is defined for numeric values (0 and 1). Therefore, when applying this format to the answer variable in PROC PRINT, it should not have a dollar sign, it should be format answer Syn.; instead of format answer $yn.;.
The correct output based on the given code, assuming the error with the dollar sign is corrected, would be as follows:
* For numeric values 0 and 1, 'No' and 'Yes' would be displayed, respectively.
* For any other numeric value, 'Unknown' would be displayed.
* For character values, the format would not apply and the actual values would be displayed.
Given that the values in the 'answer' column are numeric, the program will format them according to the defined Syn format. Thus, the report corresponding to Option D, which shows numeric values unformatted, would be the one created. This indicates that the format was not applied because of the syntax error with the dollar sign.
References:
* SAS 9.4 documentation for the FORMAT procedure and statement: SAS Help Center: PROC FORMAT

In SAS, the FORMAT procedure (PROC FORMAT) allows users to create custom formats that can be used to change the way data is displayed without altering the data itself. The VALUE statement within PROC FORMAT is a key component of this procedure.
The VALUE statement can indeed create both numeric and character formats, which is why option C is correct. Numeric format names typically do not start with a $ sign (this is reserved for character format names), and the keyword UNKNOWN is not a part of the VALUE statement syntax; it is used in a different context within PROC FORMAT, specifically for handling undefined data values when using custom formats.
Moreover, the LIB= option is not used within the VALUE statement. The LIB= option is used at the PROC FORMAT level to specify the library where the formats should be stored or from which they should be read.
References
* SAS 9.4 documentation for the PROC FORMAT and VALUE statement.
* "SAS Formats and Informats." in "Step-by-Step Programming with Base SAS 9.4" from the SAS Institute.