Some Challenge questions are from outside of Sololearn course material. What's the best way to prepare for Challenge questions?
Well, you'll have to do a lot of them, that's not a secret. However, it would be good to review the answers and try to understand the concepts and logic behind the problem, especially for those you haven't encountered before. So, learn the syntax, learn language specific concepts and well, have fun. Don't worry if you are not good at first, it's ok and it will come to you. 👍
The most profitable way for you is just to study and practice the language. Gradually and naturally your correct answer quota will increase.