A: Where's Helen?
B: She . . . . . . . . in the office.

Solution(By Examveda Team)

Statement 'A' asks about the current whereabouts of Helen. Thus, the answer must be in present continuous tense. Also, Helen must have an action and so the answer mus be a verb.
Option A is correct. It is a verb in present continuous tense. It tells us where Helen currently is and what she is doing. It says that 'She is sitting' and thus, it is correct. Here 'is' makes the gerund 'sitting' into a complete verb. Simply 'sitting' is an incomplete verb and does not make sense in the sentence.
Option B is incorrect. 'Sit' is not in present continuous tense and so it does not tell us the current whereabouts of Helen.
Option C is incorrect. 'Sitting', although a verb in present continuous tense, is an incomplete verb. 'She sitting' does not make sense. 'Sitting' is incorrect unless accompanied by an auxiliary verb.
Option D is incorrect. 'Sits' is in simple present tense and not present continuous and so it does not state the current action of Helen.