In the context of the PROC MEANS procedure in SAS, the CLASS statement is used to specify categorical variables whose unique values group the data before calculating statistics. This allows PROC MEANS to calculate statistics like the mean, sum, or standard deviation for each class or group of data defined by the CLASS variables.

The CLASS statement works as follows:

class variable-list; specifies one or more variables to define groups for analysis.

When used, PROC MEANS computes the requested statistics for each level of the variables listed, which can be very helpful for analyzing how groups compare across different statistics.

The other options are not used in the PROC MEANS procedure to specify grouping of data:

B . GROUP is not a valid statement in PROC MEANS.

C . SUMBY is not a valid statement in SAS.

D . VAR is used in PROC MEANS to specify the analysis variables for which statistics are to be computed, not to group the data.