No sooner (1)/ I had spoken, (2) than he left. (3)/ No error. (4)

Answer

2

Explanation

had I will replace I had because -
When a negative Adverb/Adverbial Expression (no sooner, hardly/barely/scarcely), is used at the beginning of a clause, it is followed by Aux. Verb + Subject
Look at the examples given below:

Hence, had I spoken is the right usage