※This sample exam is different from those that appear in the actual OSS-DB Exam.
To check which users are running sessions, such as a psql session, use the system information functions current_user, session_user, or user. Normally, the results from these are the same, but you can change the result of current_user or user by executing SET ROLE.
Although they are functions, they are treated differently and may be used without parentheses (parentheses are an error).
Therefore, the correct answer is D.