I think the answer would be : reservar, because someone is asking Alejandro if he can reserve it.
Los amigos son simpáticos.
Los because you can see amigos has an O and not an A ; also simpáticos has an O and not an A which indicates it is masculine and Los is masculine, Las is not
For most adjectives, the endings are like this:
o : masculine singular
os : masculine plural
a : feminine singular
as : feminine plural
La abuela means the grandmother. It is a feminine singular word. Therefore, you use the ending -a, so the answer is vieja.
Also, keep in mind that the adjective endings match noun endings. Abuela has the ending -a, so the adjective should also end in -a.
