A, CD, GHI, ?, UVWXY
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
first letter of the second term and a gap of two letters between the last letter of the second term and first letter of the third term.
So, the first letter of the required term would be four steps ahead of the last letter of the third term.