For this case we can translate the sentence given as:
What___________ in the market yesterday?
The sentence must be completed with the proper conjugation of the verb "buy" ("comprar" is Spanish)
From the simple perfect preterite we have:
Yo compré
Tú compraste
él/ella/Ud. compró
nosotros compramos
vosotros compramos
ellos/ellas/Uds. compraron
The sentence is in the second person singular, so the correct option is "compraste"
Answer:
Qué compraste en el mercado ayer?