Answer:
55%, or C.
Explanation:
There are 20 marbles in the bag. 9 of them are red, and 11 of them are another color. Percentages consist of a part of 100, so you would have to make your denominator (20) into a 100. To do this, simply multiply the 20 by 5. Since you did this with 20, you have to do it with your 11 non-red marbles as well. This would be 55. Put the two together, and you have 55/100. What is 55/100 in percentage form? 55%!
Hope this helped! :-D