The correct answer is: D. No error
The sentence “The President had hardly spoken a few words when the microphone stopped functioning” is grammatically correct. The word “hardly” is used to indicate that the President had only spoken a few words before the microphone stopped working. The word “when” is used to introduce a clause that describes the event that happened after the President spoke. The word “functioning” is the past participle of the verb “function,” which means to work properly. In this sentence, the past participle is used as an adjective to describe the microphone.
The other options are incorrect because they contain grammatical errors. Option A is incorrect because the word “had” should be followed by a past participle, not a noun. Option B is incorrect because the word “a” is not necessary before the word “few.” Option C is incorrect because the word “when” should be followed by a clause, not a noun.