3.
Your Answer is
Correct Answer is C
Explanation
The underlined part is the subject part of the sentence, and the correct subject form is required, so C is correct.
Item A: The leading word of the clause, so that the sentence does not have a complete subject-predicate-object, only the clause, wrong.
Item B: The problem is the same as item A.
Item D: meanwhile is a related adverb, making this sentence without a subject.