The correct answer t this question is C: "My friend Joe, who has a cast on his leg, is going to the arcade with me today." If that becomes incorrect, i am very sorry.
Answer: My friend Joe, who has a cast on his leg, is going to the arcade with me today.
Explanation: The relative clause "who has a cast on his leg" is non-defining, i.e., it gives additional information that is not essential in defining the subject. Non-defining clauses always go between commas. All the other punctuation options are incoherent: no punctuation in the first option, enclosing just the prepositional phrase "on his leg" between commas in the second option and enclosing an arbitrary portion of the sentence in the last option.
