Select the related word/letters/number from the given alternatives.
EF : OP : : ? : ST

Let's analyze the positional values of the letters in the English alphabet:
E=5, F=6
O=15, P=16

The relationship between the first pair is:

E(5)+10=O(15)
F(6)+10=P(16)
Applying the same logic to the second pair where the result is ST:
S=19, T=20

Let the missing letters be X and Y:

X+10=S(19)⟹X=19−10=9⟹I
Y+10=T(20)⟹Y=20−10=10⟹J
Thus, the missing letters are IJ.