Question

Should the response be yes or no?

Answer 1

Hi,
Mes nièces l'a appelé hier matin. (Jacques).Incorrect

Correct is : Mes nièces l'ont appelé hier matin.
Mes nièces = féminin/pluriel

Answer 2

The sentence is incorrect.

The verb (avoir) must be conjugated to agree with the noun (Mes nièces). Since "Mes nièces" is third-person plural (they - elles), "avoir" should be conjugated to "ont" ("a" corresponds to third-person singular, or "he/she/it").