In which kind of expression would you use the FORMAT statement in SAS?

The FORMAT statement in SAS is primarily used to define how variable values are displayed in the output. This is essential when you want to enhance the readability of your data, for example, by displaying dates in a recognizable format, formatting currency values, or adjusting the appearance of numeric data. By applying formats, users can control the presentation of data without altering the underlying values, making the formatted output more interpretable.

When it comes to defining variable attributes, the FORMAT statement plays a critical role because it allows you to associate a specific format with a variable. This association ensures that whenever the variable is displayed in reports or output, it appears according to the specified format.

While other options pertain to important concepts in SAS programming, they do not specifically involve the use of the FORMAT statement. For instance, performing arithmetic operations and manipulating string data involve computations and data transformations that do not require formatting for display purposes, and controlling data input relates to how data is read into a SAS dataset rather than how it is presented. Therefore, option A is the most appropriate choice for the use of the FORMAT statement.