La is the direct object pronoun that matches the third person singular in the simple present. A direct object pronoun replaces a direct object, that is a noun that directly receives the action of a verb in a sentence. Moreover, direct object pronouns in Spanish agree in number and gender with the nouns they replace. The direct object in this problem is la pelota and given that this is feminine singular we choose to replace this by la. Therefore:
You ask HOW to do this???? Simply follow the instructions as provided at the top: 'Conjugar en el pretérito los siguientes verbos.' (Conjugate the verbs into the simple past tense.)
