Answer:
It is because it is improper English
Explanation:
The correct way is "Why are you not saying anything?"
El because they're talking about one specific person out of the group
<h3>Tatiana, ¿conoces a las hermanas de Andrés?</h3><h3>Tatiana: Sí, yo _____ conozco.</h3>
Answer: D) Tatiana: Sí, yo las conozco.
Tatiana, do you know Andres's sisters?
Tatiana: Yes, I know them.
Explanation: The direct object of the verb "conocer" is "las hermanas". The direct object pronoun must agree in gender and number. Feminine, plural, "las"