Solution:WELCOME is coded as ZHOFRPH;
i.e. the letters in the code corresponding to each letter in the given word is found by adding 3 in each letter.
So, DUSSRDFK will be the code of : ARPROACH;
Because A + 3 = D, R + 3 = U, P + 3 = S, P + 3 = S, O + 3 = R, A + 3 = D, C + 3 = F, H + 3 = K