Final answer:
In SAS, the RETAIN statement is used to store values in variables across iterations in a DATA step. This is especially useful for retaining information when processing data line by line or in situations where the value should persist beyond the current iteration.
Step-by-step explanation:
In SAS, the keyword used to store values in variables is d) RETAIN. The RETAIN statement is used to hold the value of variables across iterations in a DATA step, which means that the variable retains its value from one iteration of the DATA step to the next. It's important to understand that without the RETAIN statement, variables that are not part of the incoming dataset are set to missing at the beginning of each iteration of the DATA step.
The other keywords listed serve different purposes in SAS. For example, INPUT is used to read data into a SAS dataset, FORMAT specifies the format of the data, and DEFINE is not actually a SAS keyword but might be confused with other statements such as defining variables in PROC REPORT. Therefore, for the specific task of storing values across iterations, RETAIN is the keyword that accomplishes this in SAS.