Answer:
The correct option is who.
Step-by-step explanation:
Relative pronouns are the ones used to refer to nouns mentioned previously within a sentence. The most common relative pronouns include who, whose, which, whom, and that.
Besides joining two sentences, relative pronouns are used to add information within a sentence, either defining (essential information about the mentioned previously) or non-defining (not essential to the sentence's meaning).
Regarding this, the relative pronoun "which" is used to introduce non-defining information, while the relative pronoun "Who" is used to introduce defining relative information.
Bearing in mind the stated above, it could be said that the appropriate pronoun in the sentence provided is who. Resulting in the sentence as follows: Steve made friends who later worked with him.