Solution:

we know that associate degree adverbial could be a word accustomed modify a clause or a verb. Hardly acts as associate degree adverbial within the given sentence. The given sentence is within the tense. we should always understand that once adverbials area unit accustomed report a past event and area unit placed at the start of the sentence, the topic and also the auxiliary interchange their positions. The auxiliary comes at the initial position, followed by the topic then the verb. we have a tendency to observe that the underlined a part of the sentence is following the rule. Hence, possibility D is that the correct answer.
In the initial possibility, we have a tendency to observe that the topic precedes the auxiliary, that is, auxiliary isn't at the initial position. Hence, possibility A isn't the right answer.
In the second possibility, the pluferfect tense tense is absent. The pluferfect tense tense should be used because it precedes the past event of the phone ringing. therefore choices B is wrong.
In the third possibility, straightforward tense has been used. The pluferfect tense tense should be used because it precedes the past event of the phone ringing. Hence, possibility C is wrong.