A "park trailer" is defined as having a gross trailer area not exceeding __________ sq. ft. in set up mode.

A park trailer is specifically designed for temporary occupancy and is built to be transported by a vehicle, but it is intended for use as a residence in a camping or recreational area. The definition of a park trailer is tied closely to its maximum gross trailer area when it is set up. In the context of the question, the standard for the maximum gross trailer area of a park trailer does not exceed 400 square feet in set-up mode. This limit helps maintain safety and usability standards while allowing for sufficient space that can comfortably accommodate occupants. This makes the classification of park trailers distinct from other types of trailers or mobile homes that may have larger occupied spaces.

The other options exceed this 400-square-foot limit, which is why they do not fit the official definition of a park trailer.