Question

La abuela es _______ options: viejo , vieja, viejos, viejas

Answer 1

The answer is not viejo, viejos, viejas its: vieja

Answer 2

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.