The correct answer is A. since.
The word “since” is used to indicate a point in time in the past from which something has been happening or continuing. In this case, the speaker is saying that they have not seen the person since last Sunday. This means that they have not seen the person at any point in time after last Sunday.
The other options are not correct because they do not indicate a point in time in the past from which something has been happening or continuing. The word “before” is used to indicate a point in time in the past before which something happened. The word “from” is used to indicate a point in time in the past from which something is happening or continuing. The word “about” is used to indicate a point in time in the past that is approximately correct.