The macro MP_ONE is defined to take a parameter pname with a default value of 'print'. The macro inserts the pname parameter into a PROC statement to run a procedure on the SASHELP.CLASS data set. When %MP_ONE(means) is called, proc means data=SASHELP.CLASS; run; is executed, which is valid and calculates summary statistics for SASHELP.CLASS. When %MP_ONE() is called with no arguments, the default pname value 'print' is used, resulting in proc print data=SASHELP.CLASS; run;, which is also valid and displays the data. Thus, both PROC MEANS and PROC PRINT execute successfully.