In a mixed letter coding system, if 'CAT' is coded as 'XZY', and 'DOG' is coded as 'ABC', what is the code for 'TAD'?

Answer: B. XAB
By analyzing the codes, 'TAD' can be derived from the pattern of 'CAT' and 'DOG'. 'TAD' corresponds to 'XAB' based on the coding rules established.