Three of the following four are alike in a certain way and so form a group.…

Concept: In a classification (odd-one-out) puzzle, three of the given terms share one common defining attribute and one term does not share it; the term lacking that shared attribute is the one that does not belong to the group.

Application: Check the gender denoted by each term. Nephew denotes only a male relative, Mother denotes only a female relative, and Brother denotes only a male relative — each of these three words fixes one particular gender. Cousin, however, can refer to either a male or a female relative, so its gender is not fixed by the word itself.

• Nephew — denotes a male relative only (gender fixed)

• Mother — denotes a female relative only (gender fixed)

• Brother — denotes a male relative only (gender fixed)

• Cousin — can denote either a male or a female relative (gender not fixed)

Result: Since Nephew, Mother and Brother each fix a single gender while Cousin does not, Cousin is the term that does not belong to the group.