I have not seen him . . . . . . . . last Sunday.

Answer: Option A

The correct preposition to refer to a point of time in the past up to the present is "since".

"Last Sunday" is a specific point in time, and "since" is used with specific points in the past to indicate continuity until now.

"Before" indicates a time earlier than a reference point, but does not express continuity.

"From" is used with a range (from...to...), which is not the case here.

"About" is used for approximation, not for referring to time duration.

Therefore, the correct sentence is: "I have not seen him since last Sunday."

Hence, Option A is correct.