The correct option is C When I went
Let’s examine the sentence: “I don't remember exactly when did I go to Simla.” The subordinate clause of the sentence, “when did I go to Simla,” is in simple past tense. So, it requires a verb of simple past tense. The past tense form of the verb ‘go’ is went.’ Hence, we can simply use ‘went’ in place of ‘did go’ in the subordinate clause to improve the sentence. Hence, the improved sentence would be, “I don't remember exactly when I went to Simla.” Hence, Option C is the correct answer.