Marsupials are mammals that carry and nurse their young in a:

Marsupials are a distinctive group of mammals known for their unique reproductive strategy. They give birth to relatively undeveloped young, which then complete their development in a specialized external structure.

• Pouch (C) is correct. Marsupials, such as kangaroos and koalas, are characterized by the presence of a marsupium, or pouch, where the young (joeys) continue to develop and nurse after birth.
• Nest (A) is incorrect; nests are typically built by birds or some rodents for shelter and raising young, not a defining feature for marsupial development.
• Burrow (B) is incorrect; burrows are underground tunnels used for shelter by many animals, but not specifically for marsupial young development.
• Den (D) is incorrect; a den is a natural or artificial shelter for wild animals, often used by carnivores, but not the specific structure for marsupial young.