If ' LIGHT’ is coded as ‘QFDEY', ' PLUM' is coded as 'UIRR', and then 'FATHER' is coded as '____'.

The logic followed here is that the first and last letter of the word is replaced by 5th succeeding letter and all the middle letters of the word are replaced by 3rd preceding letter as per English alphabetical series.

In LIGHT,

L + 5 = Q, I – 3 = F, G – 3 = D, H – 3 = E, T + 5 = Y

Code for LIGHT is QFDEY

Similarly,

In PLUM,

P + 5 = U, L – 3 = I, U – 3 = R, M + 5 = R
Code for PLUM is UIRR

Similarly,

In FATHER,

F + 5 = K, A – 3 = X, T – 3 = Q, H – 3 = E, E – 3 = B, R + 5 = W

Code for FATHER is KXQEBW