Question

before thermal energy in fuel can be transferred to the air in heating systems, what has to occur?
its temperature must be raised by converting thermal energy to chemical energy.
the energy in its chemical bonds must be converted to thermal energy.
the kinetic energy must be stored in the chemical bonds of the fuel.
its temperature must be lowered to transfer energy from the air to the fuel.

Explanation:

Fuels contain chemical energy in their bonds. For thermal energy (heat) to be transferred to the air in heating systems, the chemical energy in the fuel's bonds must be converted to thermal energy. Converting thermal energy to chemical energy (as in the first option) is incorrect as it's the opposite of what happens in fuel combustion. Kinetic energy being stored in chemical bonds (third option) is wrong because chemical bonds store chemical energy. Lowering the fuel's temperature to transfer energy from air to fuel (fourth option) is against the principle of heat transfer (heat flows from higher - temperature to lower - temperature regions, and we want heat from fuel to air).

Answer:

The energy in its chemical bonds must be converted to thermal energy.