Answer : 3. "A large pizza was being eaten by him."
Explanation :

In Active Voice, a sentence emphasizes the subject, performing an action.
In Passive Voice, a sentence emphasizes the action or the object of the sentence.
The given sentence is in the active voice and 'He' is the subject and 'a large pizza' is the object.
When we convert this sentence into passive voice, the subject 'He' of the active voice becomes the object 'him', the object 'a large pizza' becomes the subject.
The passive format "was + being + V3 (eaten)" should be used.
This is the active and passive voice rule for the past continuous tense.
Hence, the correct answer is option 3.