Choose the word which is different from the rest.

The answer is B. Roof.

A den is a shelter or resting place for animals, typically a cave or hollow tree. A burrow is a hole or tunnel dug in the ground by an animal for habitation or as a means of escape. A nest is a structure built by a bird or other animal for laying eggs and sheltering its young. A roof is the covering of a building or other structure that protects it from the weather.

Roof is the only option that is not a place where animals live. The other options are all places where animals live or build their homes.