To store all calculated means and standard deviations in a SAS data set, which statement is necessary?

The chosen answer correctly identifies the syntax required to store statistical summaries such as means and standard deviations into a SAS dataset. The output statement, when used in procedures such as PROC MEANS or PROC SUMMARY, allows you to specify the name of the output dataset, as well as the statistics you want to capture and their respective variable names.

In this context, "output out=RESULTS" directs SAS to create a new dataset called RESULTS where the calculated means (denoted by m1, m2) and standard deviations (denoted by s1, s2) will be stored. This encapsulates the results of the statistical calculations performed in the procedure, allowing for easy access and manipulation of these values in subsequent analyses.

The other choices do not conform to the correct syntax for creating an output dataset containing calculated statistics. Options that use terms like "store," "save," or "create data" do not align with the established SAS procedure conventions for outputting calculated statistics, thus making them invalid for this purpose.