The plural would be the third one c los autobuses
I'm not exactly sure what you're asking, but I'll try.
The answer is "compro" because it's past tense. I should know, I speak both Spanish and English.
Hacemos regla de 3
10 cajas 8 horas
X
25 cajas x
x= 25cajas * 8 horas
10 cajas
x=200 dividir por 10=20 horas
