Select the related word/letters/number from the given alternatives.
CDE: XWV::?: QPO

Let's look at the relationship in the first pair: CDE:XWV.
The letters are pairs of opposite letters from the start and end of the alphabet sequence (A↔Z, B↔Y, etc.):

C↔X (3rd from start / 3rd from end)

D↔W (4th from start / 4th from end)

E↔V (5th from start / 5th from end)

Now, we need to find the opposite pairs for QPO:

Q↔J (17th from start / 17th from end)

P↔K (16th from start / 16th from end)

O↔L (15th from start / 15th from end)

Thus, the corresponding set of letters is JKL.