75%=) I think thats probably a C=)
This would depend on the grading curve. You missed 15 questions out of 60. That is 25% missed 75% correct. Most grading scales I have seen that would be a low C. But again, it all depends on the scale you school uses.
Low or high a C is a C and it sounds like a C and you just missed a D by a hair...the best to you.