Which of the following pronoun completes the sentence correctly?

Each of the boys began to clean _____desk.
a. his
b. him
c. their***

2 answers

No. What is the subject in this sentence. It's not "boys". "Boys" is the object of the preposition "of". What does the adjective/prepositional phrase modify? Is it singular or plural?
if anyones wondering it should be "his"