Answer: C. Chicas
Explanation:
When you fill the blank with chicas, it looks like this: ¿Cuántas chicas? You would not use any other term because the word cuántas is plural and feminine.
If it was a masculine or neutral term, it would be "¿cuántos?".
If it was a singular term, it would not exist because the word "cuántas" or "cuántos" means "how many".
Therefore, the answer is C.