In which sentence is the underlined verb in the active voice? The house will be painted by my parents. My parents painted the house that summer. The house was painted that summer. The house had been painted by my parents.

The answer to your question would be that the sentence in which the underlined verb is in the active voice is the following one: My parents painted the house that summer.

In most English sentences with an action verb, the subject performs the action denoted by the verb. Since the subject does or acts upon the verb in these senteces, the sentences are said to be in the active voice. In this case, the NP subject "the parents" stands for the ones doing the action (painting the house).