Làpiz is a masuline word in Spanish, so "la" and "una" cannot be used since those are feminine. "Los" is also incorrect since that is the plural form, which is not being used in the sentence. Therefore, un is the correct choice.
The answer for the first one is D. vas. The answer for the second one is d. cierra. I hope this helps. I'm sorry if I get one wrong. I don't think I did.
