He will tell you (1) / about it when (2) / he will come back. (3) / No error (4)

Options

A. 1 He will tell you
B. 2 about it when
C. 3 he will come back.
D. 4 No error

Answer

3

Explanation

he comes back will replace he will come back because -
will is not used in Conditional clauses and the Verb is in Simple Present Tense
Look at the examples given below:
She will do it, when she comes homes. I will leave all my money to charity, when I die,
Hence, he comes back is the right usage