In the pair CAB : XZY every letter is replaced by the letter that stands as far from Z as the original stands from A. Complete the analogy: MUD : ?

M is 13th from A, so its partner is 13th counting back from Z, which is N. U is 21st from A, and 21st from Z is F. D is 4th from A, and 4th from Z is W. That gives NFW. NFV slips by one on the last letter, since V is 5th from Z and not 4th. MFW leaves M standing, as though M were its own partner. WFN is the right three letters written back to front.