21.
Your Answer is
Correct Answer is D
Explanation
Item D: It is the omission of the adverbial clause. Before the omission, it should be:...as if (she was) waiting for someone. [The adverbial clause is the same as the subject of the main clause, and when there is a be verb in the clause, the subject and be verb of the clause can be omitted]
Item A/B: The pronoun is wrong, and the subject of the verb here should be she. Here it means "as if she was waiting for someone".
Item C: wrong tense.