The answer is C. This is because the red paint must be proportional to the yellow paint, and the proportion given to us is 6/8 (or 6:8 as a ratio). We notice that when simplified, 6/8=3/4, meaning that for every 3 parts of red, there are 4 parts of yellow. By multiplying 3/4 by a certain number of parts for yellow, we'll get the proportionate parts for red.
For example, if you wanted to find out the proportionate part of red for 16 parts of yellow, you would do 3/4*16, which is 12, so that would be 12 parts of red.
