Final answer:
The term not a fixture is D. riding lawn mower.
Step-by-step explanation:
A fixture is a permanent feature or element that is attached to a property and cannot be easily removed. Heating systems, bathroom cabinets, and light fixtures are all examples of fixtures because they are fixed in place and not intended to be easily moved. On the other hand, a riding lawn mower is not a fixture because it is a movable item that is not permanently attached to a property. It can be easily transported and used in different locations. The correct answer is D. riding lawn mower.