The correct answer is: D. I saw the rehearsal being conducted by him.
The other options are incorrect because they do not accurately reflect the meaning of the sentence “I saw him conducting the rehearsal.”
Option A, “He was seen conducting the rehearsal,” is incorrect because it implies that someone else saw him conducting the rehearsal, but the sentence does not specify who saw him.
Option B, “I saw the rehearsal to be conducted by him,” is incorrect because it implies that the rehearsal was already planned to be conducted by him, but the sentence does not specify whether or not the rehearsal was planned in advance.
Option C, “He was seen by me to conduct the rehearsal,” is incorrect because it implies that the speaker saw him conducting the rehearsal, but the sentence does not specify whether or not the speaker was the only person who saw him.
Option D, “I saw the rehearsal being conducted by him,” is the correct answer because it accurately reflects the meaning of the sentence “I saw him conducting the rehearsal.” The sentence states that the speaker saw him conducting the rehearsal, and it does not specify who else saw him or whether or not the rehearsal was planned in advance.