I saw him conducting the rehearsal.

Let's break down how to change this sentence from active to passive voice!

First, identify the key parts of the sentence: "I saw him conducting the rehearsal."
* Subject: I (the one doing the seeing)
* Verb: saw
* Object: him (the one being seen)
* Participle Phrase: conducting the rehearsal

When changing to passive voice, the object of the active voice becomes the subject of the passive voice. So "him" will become "he."

Also, the verb 'saw' need to be converted into passive format according to the tense (simple past form) .

Now, let's look at why the correct answer is Option A: He was seen conducting the rehearsal.

* "He was seen" correctly uses the passive form of the verb "saw." It's in the past tense and shows that the action was done *to* him.
* "conducting the rehearsal" This part remains same as it is participle phrase.

Why the other options are wrong:

* Option B: I saw the rehearsal to be conducted by him. This changes the meaning and isn't a direct passive transformation of the original sentence.
* Option C: He was seen by me to conduct the rehearsal. While grammatically correct, adding "by me to conduct" makes the sentence unnecessarily complex and doesn't flow as naturally as Option A. it changes the meaning also.
* Option D: I saw the rehearsal being conducted by him. This is in active form not in passive form.

In summary, the simplest and most accurate passive form of the original sentence is "He was seen conducting the rehearsal."