Mammals that give birth to under-developed young that must continue developing in a pouch-like structure are termed
a. monotremes.
b. marsupials.
c. placentals.
d. bird-like.

The answer is b. marsupials. 

Monotremes, marsupials, and placentals are groups of mammals that mainly differ in modes of reproduction.
Marsupials are the group of mammals that give birth to under-developed young. The young continue their development inside a pouch-like structure. The most famous marsupials are kangaroos, koalas, opossums, numbats, etc.

Monotremes are the group of mammals that lay eggs, so they do not give birth to live young.
Placentals are mammals that give birth to fully developed young. The young develop inside a mother's uterus.