Hey there! I'm happy to help you out!
Let's think of our original price ($P) as 100%. This is also the same thing as 1 because 100% is one whole thing.
Now, we see that he got a 15% off discount. Now, we just subtract 15% from 100% to see what percent he actually payed.
100%-15%=85%
This means that $R is 85% of $P (100%).
Answer A is not correct because 0.85% is less than 1%, which is not 85%.
Answer B is not correct either because 0.15% is also less than 1%.
Answer C is correct because $R is 85% of the original cost.
I hope that this helps! Have a wonderful day!