He also defeated Buggy. Even if Buggy wasn't a Warlord back then, maybe that question asks for those who became Warlords afterwards. This would mean that the correct answer was 4 (Buggy, Crocodile, Gecko Moria and Doflamingo).
For the second question, I think that it doesn't necessarily ask for the five Straw Hats with the highest bounty, but rather for the correct order of the crew members mentioned in each answer. In this case, the answer "Luffy, Jinbe, Usopp, Robin, Nami" is the only answer where the five Straw Hats who are actually mentioned are in the correct order, while Zoro and Sanji aren't mentioned in that particular answer.
I'm pretty sure that Zoro and Sanji's bounties were higher than Usopp, Robin and Nami.
For the second question, I think that it doesn't necessarily ask for the five Straw Hats with the highest bounty, but rather for the correct order of the crew members mentioned in each answer. In this case, the answer "Luffy, Jinbe, Usopp, Robin, Nami" is the only answer where the five Straw Hats who are actually mentioned are in the correct order, while Zoro and Sanji aren't mentioned in that particular answer.