The correct answer is option 4 - Platypus. The platypus is a unique mammal known for its ability to lay eggs. It is one of the few mammals that belong to a group called monotremes, which also includes the echidna. Monotremes are a primitive group of mammals that lay eggs instead of giving birth to live young. This is different from most mammals, which give birth to live offspring.
Options 1, 2, and 3 are incorrect. Bats, whales, and weasels are all mammals, but they do not lay eggs. Bats and whales are classified as placental mammals, which means they give birth to live young after a gestation period. Weasels, like many other mammals, also give birth to live young.