Answer: 4. At the skyscraper, we watched them in action building their masterpiece.
Explanation:
Active voice is when a form of a verb is such that the subject is the one that's performing the action and then takes a direct object.
The sentence in the paragraph that is in the active voice is "At the skyscraper, we watched them in action building their masterpiece".
The subject here is "we" and it takes a direct object " them". This is the correct option as others aren't in active voice.
