The correct answer is D. He was seen leaving the house by me.
A. He had been seen leaving the house. This is incorrect because it implies that he was seen leaving the house at some point in the past, but you are saying that you saw him leaving the house right now.
B. He was seen to be leaving the house. This is incorrect because it implies that someone else saw him leaving the house, but you are saying that you saw him leaving the house.
C. Leaving the house he was seen by me. This is incorrect because it is grammatically incorrect. The correct way to say this would be “I saw him leaving the house.”
D. He was seen leaving the house by me. This is the correct answer because it is grammatically correct and it accurately reflects what you are saying. You are saying that you saw him leaving the house.