A "kangaroo word" is a word that hides a smaller word inside its own letters, with the letters of the smaller word appearing in the same order as they do in the bigger word, though not necessarily right next to each other. The hidden smaller word must also carry roughly the same meaning as the full word, similar to how a baby kangaroo, called a joey, is carried inside its mother's pouch.
"Masculine" is a kangaroo word because it hides the word "male" inside it, using letters in the same order: M-A-S-C-U-L-I-N-E gives us M, then A, then, skipping ahead, L, then, skipping further, E, spelling M-A-L-E. Since "male" means essentially the same thing as "masculine", this satisfies both requirements of a kangaroo word, a hidden smaller word in the correct letter order and a matching meaning.
The other options do not qualify. "Woman", "Man", and "Child" are already short, simple words, and none of them hides an even smaller synonym spelled out in the same letter order within their own spelling. Since only "masculine" satisfies the strict definition of hiding a smaller, same-meaning word inside itself, option 1 is correct.