It was him (1)/ who came running (2)/into the classroom. (3)/ No error. (4)

Answer

1

Explanation

he will replace him because - It was him is the Subject of the sentence and in the Subject, Nominative Case (he) will be used.
Look at the examples given below:
`underset("Subject")underset(darr)("Him")`go to the store to buy milk.
`underset("Subject")underset(darr)("He")` goes to the store to buy milk.
Hence, It was he is the right usage.