Explanation:

- Several foreign students have been asked by the US to leave the university campus and ______ home.

- Option 1: "returned" – This past tense form is not suitable as it doesn't match the ongoing request context in the sentence.

- Option 2: "returning" – Present participle form doesn't fit here. The sentence demands an infinitive verb.

- ? Option 3: "return" – This is the base form of the verb, and fits as the infinitive "to return" is understood in the context "asked to return." It correctly completes the sentence.

- Option 4: "returns" – This present tense form is not appropriate because it doesn’t follow the grammar structure needed here.