Correct option is A
SUNSET is coded as 'TTOTDU'
Logic: Alphabets are increasing by 1 place and vowels decreasing by 1 place .
S + 1 = T, S + 1 = T
U - 1 = T, T + 1 = U
N + 1 = O, A – 1 = Z
S + 1 = T , R + 1 = S
E - 1 = D, S + 1 = T
T – 1 = S,

So, TUZST is correct answer.
Thus, correct option is (a).