Hello!
your answer is: B. C + 9
If you currently have C cars, then that stays in the mathematical sentence no matter what.
9 more means you are adding 9 to the current amount. In this case, C is the current amount.
therefore, you add 9 to C
made into a mathematical sentence, you get C + 9, or 9 + C
I hope this helps, and have a nice day!