What does okonkwo consider to be the most important mark of a successful man in his village?
being wealthy
being strong
having several wives and lots of children
being a good musician

I would say the correct answer is "having several wives and lots of children". Much of Okonkwo’s behavior results from a desire to be a complete opposite of his father. That means Okonkwo attempts to work hard, provide for his family materially, be brave etc. As a result he becomes successful in many ways: has three wives, he's rich etc. He is also known for being a very good warrior. All this attributes to his success, so we can see that the first three answers make sense, but if I had to choose one it would be "having several wives and lots of children", because other things would naturally come. If you are rich, women will want you, and the same goes if you are the best warrior.