Diagram C represents 75%.
In Diagram C, since there are 15 boxes filled out of 20 boxes, the fraction is

. Simplify

and express that fraction as a decimal. To turn a decimal into a percentage, mulitply the decimal by 100.

%
Another way to solve is to think of the diagram equal to 100%. In Diagram C, each box equals 5%. This would make all 20 boxes together equal 100%. Since 15 boxes were filled, and each box represents 5%, you can multiply 15 by 5.
15 x 5 = 75