The correct answer is C. at.
The word “at” is used to indicate a location or position. In this case, the speaker is disappointed that the doctor is not in his seat. The seat is a specific location, so the word “at” is the most appropriate choice.
The other options are not as accurate. The word “on” is used to indicate a surface or area. The doctor’s seat is not a surface or area, so the word “on” is not a good choice. The word “over” is used to indicate a position above or across something. The doctor’s seat is not above or across anything, so the word “over” is not a good choice. The word “from” is used to indicate a starting point or origin. The doctor’s seat is not a starting point or origin, so the word “from” is not a good choice.