C is your answer because compras means you buy
I will answer this by translating it.
I : Yo - Ar= o Er= o Ir= o
You : Tú. - = as = es = es
He/she/it : Él/Ella - = a = e = e
We : Nosotros - = amos = emos = imos
You(pl) : Vosotros - = áis = éis = Ís
They : Ellos - = an = en = en
