Kangaroo words are an unusual category of English word puzzles. A word qualifies as a kangaroo word when a smaller word that means roughly the same thing can be spelled out using letters taken from the larger word, in the exact order they appear, without needing to be consecutive. The name comes from the image of a kangaroo carrying its young inside its pouch, the smaller word riding along inside the bigger one.
Testing "masculine" against this rule works out cleanly. Spell out the letters: M, A, S, C, U, L, I, N, E. Picking out M first, then A, then jumping ahead to L, then jumping further to E, spells M-A-L-E in that exact sequence. "Male" is a near-perfect synonym for "masculine", so this word passes both tests a kangaroo word needs to pass, a smaller word hidden in order and a matching meaning.
None of the remaining choices work this way. "Woman", "Man", and "Child" are all fairly short words themselves and do not contain an even smaller word with a similar meaning spelled out from their own letters in sequence. Because "masculine" hides "male" inside it in the right order and with the right meaning, it stands out as the true kangaroo word here, confirming option 1 as the answer.