What dose the question mean by:
which best describes the complement of choosing a red marble?
i don't understand. would the answer be:
choosing a yellow marble
yes or no, why or why not
and

By complement, I believe that's referring to the color wheel. ie Which colors go best together.

On the color wheel, green is directly opposite of red - so these two compliment each other best. We know green must be in the answer. That rules out D.

So we're being a little more flexible and also using the colors that are close to green. On the color wheel, this is yellow and blue. This rules out A, because we can't include one side and not include the other side too - that would make no sense.

So that leaves us with these 3 colors, and these 3 colors and red (or all of them). Does red complement red? Well according to the color wheel, it does not. There are colors that separate red from the others - purple and orange. So these 3 are not complimentary. 

Why doesn't a color compliment itself? That's probably more along the lines of a matter of opinion, but there are two reasons. One is when a color is by itself, it tends to fade out of our mind. Nothing about it alone 'stands out'. A complimenting color brings that figurative "pop" to make something grab your attention.

Second would be an overwhelming amount of one color. If you were to walk into a room that is solid red, that is generally considered to be a negative. As a person, you don't feel comfortable in that environment, and nothing about it is pleasing to the eye.

Long winded answer for C

20y5 times 2 is 1010 times -2 is -20, plus y