The SUM statement in SAS is used to sum values across observations, adding the value of the expression on the right side of the statement to the variable on the left. It does not use an equal sign (eliminating A), only one variable can be on the left-hand side of a SUM statement (eliminating B), and it is not initialized to 1 by default (eliminating D). What makes the SUM statement particularly useful is that it ignores missing values when summing across observations, which means that missing values do not affect the sum (option C is correct).
References:
* SAS documentation on the SUM statement, SAS Institute.

Explanation
https://v8doc.sas.com/sashtml/lrcon/z0961108.htm

Option B is correct. The DATE9. format in SAS displays dates in the ddMMMyyyy format, which corresponds to the example given (25JUN2019). This format writes dates with a two-digit day, a three-character month abbreviation, and a four-digit year. The other options do not match the correct format:
* A is incorrect because ddMMMyy9. format would display a two-digit year.
* C is incorrect because there is no such format as Ddmmmyyyy9. in standard SAS formats.
* D is incorrect because Dmy9. does not correspond to the required format.
References:
* SAS 9.4 documentation on date and time formats.