4). At the skyscraper, we watched them in action building their masterpiece.
Explanation:
As per the question, option 4 displays the sentence that is written in active voice as the subject/doer is accomplishing the action expressed by the verb in the sentence which is contrary to the sentence written in passive voice which involves a subject that receives the action or being acted upon(by the verb). Options 1, 2, and 3 emphasizes the object('the skyscraper visit', 'the museum', and 'those works of art') instead of the subject which implies that they are the recipient of the action. Thus, only option 4 contains the subject 'we' that is performing the action 'watched' rather then receiving it. Therefore, <u>option 4</u> is the correct answer.
