To answer this question, you must first understand what a fourth is. 1/4 is simply when a one is split into four parts, and you take one of those parts. Imagine you have a pie, and you cut it into four equal pieces. If you take one of those four pieces, you have one fourth of the pie. So if you have three fourths of the pie, you have three pieces out of the four total. This question is basically asking: "If you have three pieces out of four, how many pieces do you need to make one whole pie?"
Therefore, the problem looks like this:
3/4 + ?/4 = 1
and therefore the answer is 1/4 because you only need one fourth to make a whole number.