i think the answer is "XFO" bcause consider in
s,t,u,v,w 1,2,3,2,3, and "X" has to be in first position
a,b,c,d,e it follows the series in the position 2,3,1,3 and next alpha has to be in "F" in second position
j,k,l,m,n 3,1,2,1,2 the next alphabet "O" is in 3rd position.

so da ans is "XFO"