The sentence that contains an error in pronoun use is (1) "Marcus and his brothers always help his dad with maintaining the lawn every weekend."
The pronouns "his" in this sentence can lead to confusion about whose dad they are referring to. It would be clearer to replace the second "his" with "their" to indicate that Marcus and his brothers are all helping their dad. The corrected sentence would read: "Marcus and his brothers always help their dad with maintaining the lawn every weekend."