c. it should be had as given in option 3rd
Past perfect tense is used in the "if" part of a conditional to express a condition and a result. More read: https://www.grammarly.com/blog/past-perfect/
Correct option: C, had
answer:- (c) had
Had I not informed you on time, you would have missed the opportunity.
The answer is "Had". The reason is inversion concept is applicable here.