The answer is D. Escalator.
A ladder is a portable structure consisting of two long poles joined at regular intervals by crosspieces, used for reaching high places.
A staircase is a series of steps leading from one level to another.
A bridge is a structure built to span physical obstacles without closing the way beneath such as a river, valley, or road, for the purpose of providing passage over the obstacle.
An escalator is a moving staircase or moving walkway for carrying people between floors of a building or between different levels of a transportation hub.
The escalator is different from the rest because it is the only option that is powered. The other options are all non-powered structures that can be used to move people from one level to another.