Introduction:
The heating effect of current is a phenomenon in which electrical energy is converted into heat energy when an electric current passes through a conductor. This effect is utilized in various electrical appliances to generate heat. However, one of the options listed does not rely on the heating effect of current. Let's analyze each option to determine the correct answer.

Analysis of options:
a) Electric heater: An electric heater operates by passing an electric current through a high-resistance wire, which then generates heat due to the heating effect of current. This option is based on the heating effect of current.

b) Electric bulb (with filament): A traditional incandescent light bulb consists of a filament that is heated to a high temperature by the passage of an electric current. As a result, the filament emits light and heat. This option is also based on the heating effect of current.

c) Electric iron: An electric iron contains a heating element, which is usually a high-resistance wire. When an electric current is passed through this element, it heats up and transfers the heat to the iron's soleplate, allowing it to be used for ironing clothes. This option is also based on the heating effect of current.

d) Microwave: Unlike the previous options, a microwave oven does not rely on the heating effect of current to generate heat. Instead, it uses microwaves, which are a form of electromagnetic radiation, to directly heat the food. The microwaves cause the water molecules within the food to vibrate, generating heat in the process. Therefore, a microwave oven does not utilize the heating effect of current to generate heat, making it the correct answer.

Conclusion:
Among the given options, the microwave oven (option D) is not based on the heating effect of current. While electric heaters, bulbs with filaments, and electric irons all rely on the heating effect of current to generate heat, a microwave oven uses electromagnetic radiation to directly heat food.