"I have not heard it."
Translation:Je ne l'ai pas entendu.
je ne l'ai entendu. Isn't pas optional in casual spoken French? Or did I do something else wrong here?
It's actually the opposite: the 'ne' is ommitted in casual spoken French
(Je ne l'ai pas entendu -> Je l'ai pas entendu), but this ommission is not standard and thus usually not accepted on duolingo.
It depends. In this case, "it" in this sentence refers to a feminine word