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.
A Yolanda le gusta los plátanos
A nosotros nos gustan el café
A mí me gusta los frijoles
¿A tí te gusta la comida picante?
A ellos les gustan el pescado
the first one jugar al hockey
