The below given figure comprises of a pattern through which you can determine the missing letter. Can you push your mind to find the pattern and add the missing letter?

which letter replaces the question mark

The missing letter is Q.

If you add the three numbers on each square, the value corresponds to the equivalent alphabet in the center. For example 6 + 4 + 4 = 14 and N is the 14th letter. Similarly 14 + 2 + 1 = 17 or the letter Q.