A. Sounding your horn to have fun is just absurd, and could cause an issue. Either leading to other drivers to have a scare and wonder what was the problem, or they would get angry because they'd think you're trying to be rude. So, answer A. is incorrect.
B. Sound your horn to demand the right-of-way is extremely wrong, and incorrect. So, answer B. is incorrect.
C. You must sound your horn when necessary in order to avoid any collisions. You must do this to help warn and prevent another driver from getting into an accident with you. Therefore, C is correct.
D. Sound your horn to give other driver a piece of your mind. This is also known as road rage, it's not worth doing since you could get distracted while driving, and could lead to serious (sometimes fatal) issues. So, answer D. is incorrect.
So, as I said above, the correct answer is: C. When necessary to avoid collisions
Good luck with your studies, and I truly hope this helps!~
