C. is what percent of 125?

Step 1: We make the assumption that 125 is 100% since it is our output value.

Step 2: We next represent the value we seek with $x$.

Step 3: From step 1, it follows that $100/%=125$.

Step 4: In the same vein, $x/%=125$.

Step 5: This gives us a pair of simple equations:

$100/%=125 (1) $.