A, CD, GHI,?, UVWXY

Each term consists of consecutive letters in order. The number of letters in the terms goes on increasing by one at each step.

Also, there is a gap of one letter between the last letter of the first term and the first letter of the second term; a gap of two letters between the last letter of the second term and the first letter of the third term; and so on.

So, there should be a gap of three letters between the last letter of the third term and the first letter of the desired term.