Idon't know how or what one is proportion

The answer is D. and I'll explain why.  A proportion looks like this, 1/30.  Or one over thirty.  A proportion must be equal.  Like 1/30 = 3/90.  You must be able to multiply both by the same number, and then be able to scale it down to get the same thing.  I know it sounds complicated, but it's really not.  If you have any more comments, or questions, I'll be watching.

3        12
_   =    _
4        16

The answer is a 5/20 because if u add all of the marbles there are 20 and there are 5 red marbles so 5/20 is the answer mark as brainliest