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

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.

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

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