Difference of five times a number n and ten

5n - 10 is the way to write the expression you wrote in English above.
It does not mean much until we set it equal to something.
For example:
The difference of five times a number n and 10 is 25.

Now
 5n - 10 = 25
by subtracting 10 from each side we get 5n=35  and then if we divide both sides by 5, we get n=7.   

Difference means subtraction
times means multiplication

Difference of a and b would be a-b

so difference of 5 times a number, which is 5n, and 10 would be: 5n-10

So your answer is:

Cwill be your correct answer