All the sentences are correct in terms of pronoun use. However, if you are looking for which sentence may seem less clear or have an indirect connection to pronouns, it would not be specific to pronouns themselves.
That being said, if I must choose an option based solely on implied meaning or clarity, I would suggest:
sentence 1
This is because "his dad" could be seen as slightly ambiguous without confirmation that Marcus's brothers are indeed helping "his" dad.
However, it's important to note that this is not a strict case of pronoun error; rather, it can be more about clarity. If you are strictly looking for an incorrect pronoun, there is none in any of the sentences.