Final answer:
A water heater or furnace in a garage should have any source of ignition at least 18 inches above the floor to prevent fire hazards, in line with safety measures outlined in building codes.
Step-by-step explanation:
The proper installation of a water heater or furnace within a garage has specific safety requirements according to many building codes. The correct answer to the student's question is that the appliance should have any source of ignition at least 18 inches above the floor to prevent a fire hazard. This is to ensure that flammable vapors, which are heavier than air and may accumulate near the floor, do not come into contact with the ignition source. Other safety measures typically recommended include ensuring that vents and chimneys work properly, and the placement of carbon monoxide detectors in the home. Furthermore, using energy efficiently in heaters and considering alternative energy sources, such as solar heating, is beneficial for both safety and environmental concerns.