Los castillos
los acueductos
el encierro
la arquitectura de Gaudi
la monarquia espanola
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.
Familiar-pariente
Avería -daño
Seguir-continuar
