Answer the question asked based on the points. 1. A3P means A is the mother of…

In these coded blood-relation puzzles, a three-character code LnL' states that person L relates to person L' by the relation numbered n (here 3 = mother of, 4 = brother of, 5 = daughter of, 9 = husband of). When several such codes are chained end to end and consecutive codes share a letter, that shared letter is the pivot linking two relations, so a seven-character string encodes three relations at once, and any indirect relation (such as an in-law tie, which needs one blood relation plus one marriage relation) must be built up link by link.

Applying this to the code that decodes to the required relationship, M9N5K3J:

1. M9N: M is the husband of N.

2. N5K: N is the daughter of K.

3. Combining steps 1-2: since M's wife N is K's daughter, K is a parent of M's spouse - i.e., K is a parent-in-law of M.

4. K3J: K is the mother of J, so K is female.

5. Combining steps 3-4: K, being a female parent-in-law of M, is the mother-in-law of M.

Checking the other codes confirms none of them fit:

• M9N3K4J: M9N gives M is husband of N; N3K gives N is mother of K; K4J gives K is brother of J. Here K turns out to be the child of M and N, not a parent-in-law of M.

• K5J9M3N: K5J gives K is daughter of J; J9M gives J is husband of M; M3N gives M is mother of N. Here K is a child within J and M's family, with no marriage link placing K as M's parent-in-law.

• K3J9N4M: K3J gives K is mother of J; J9N gives J is husband of N; N4M gives N is brother of M. Here K is the parent-in-law of N (J's wife), not of M, since M is only N's sibling and not N's spouse.

Only the chain M9N5K3J satisfies both conditions - K is female, and K is the parent of the person M is married to - so it is the code that means K is the mother-in-law of M.