Solution

"The mason is building the wall" is in the active voice. Active voice is used when the subject of the sentence performs the action described by the verb. In this sentence, "the mason" is the subject and "is building" is the verb that describes the action the subject is performing. On the other hand, passive voice is used when the subject receives the action described by the verb. For example, if the sentence was written in the passive voice, it would be "The wall is being built by the mason." In this case, the subject "wall" is receiving the action "being built" and the actor "mason" is mentioned in the sentence, but not as the subject.