I told him (1) / that I availed (2)/ the opportunity. (3)/ No error (4)

Answer

2

Explanation

availed myself of will replace availed. avail oneself of (Idiom): take advantage of Look at the examples given below: We availed ourselves of his goodwill. Guests availed themselves of the full range of hotel facilities.
Hence, that I availed myself of is the right usage