The backslash is a zoom out key, as it forces the _____________ to zoom out to the entire length of a sequence.

The backslash key is indeed used to zoom out the timeline to show the entire length of a sequence. When pressed, it effectively adjusts the view in the timeline panel to encompass all clips and elements, allowing users to see how their entire sequence fits together at a glance. This is especially useful for editors who need to get an overview of their project, enabling them to manage and arrange clips more efficiently.

In the context of the question, the other choices do not accurately describe what the backslash key operates on. The project refers to the entire collection of media and sequences, the clip is a single piece of video or audio within the timeline, and the track is a horizontal section in the timeline that contains clips. Therefore, the timeline is the correct focus, as it is specifically the area where adjustments to zoom levels can provide a comprehensive view of the entire sequence.