Select the related word/letters/number from the given alternatives.
SAP : WET :: ? : XVI

Let us analyze the relationship between the first pair, SAP and WET, by tracking the positional values of the letters in the English alphabet:
S(19)→W(23)⟹+4

A(1)→E(5)⟹+4

P(16)→T(20)⟹+4

Applying the same pattern (−4 from the second term to find the first missing term):

X(24)−4=20→T

V(22)−4=18→R

I(9)−4=5→E

Hence, the missing letters are TRE.