When is ‘a’ used after ASISTIR in a sentence?

(I have no idea if this will be answered...)

translated the questions are:

do you need that cup?

need to use these napkins?

do you prefer this apple pie or that flan?

would you like more?

do you prefer this chocolate ice cream or that vanilla ice cream?

1. no necesito eso taza

2. sí tengo esas servilletas

3. prefiero esta tarta de manzana

4.   i'm not sure

5.prefiero este helado de chocolate

also if you need later on i used to use this website:       plz let me know if im wrong: )

i don't know spanish iam

explanation:

really sorry it's my fault but it's it a un cibercafé for sure