Mammals whose young develop in a pouch on the mother's body are called

placental mammals, monotremes, marsupials, gymnosperms

The correct answer is: marsupials.

Marsupials are actually defined by their ability to hold the young in the pouches, where they can be well protected. Some examples are Kangaroos and Koalas.

Other answers are wrong: for example, gymnosperms are plants, not animals.