Correct answer:
silla
Since before the blank we have the indefinite article that matches the feminine singular form, then we must choose a feminine singular noun. In this way, the more logical noun that matches this sentence is silla because it's a feminine singular noun that translates into chair. So, we can write this sentence as follows:
Ay, Julio, estás muy pálido. ¿Quieres sentarte en una silla?