Tú is the subject pronoun for the second person singular, so we must use the conjugation from the list that matches this person, too. So compraste is the conjugation of the verb comprar for the second person singular in the preterite tense. This tense is used to describe actions that took place or were completed at a certain point in the past. Therefore:
<em>¿Qué </em><em>compraste</em><em> tú en el mercado ayer? (What did you buy in the market yesterday?)</em>
