**Heating Effect of Current**

The heating effect of current refers to the phenomenon where electric current flowing through a conductor produces heat. When an electric current passes through a conductor, the conductor experiences resistance. This resistance causes the electrons to collide with the atoms or molecules of the conductor, resulting in the conversion of electrical energy into heat energy.

**Options:**

a) Electric iron: An electric iron utilizes the heating effect of current to produce heat. The electric current flows through a coil of wire inside the iron, which has high resistance. This resistance generates heat, which is then transferred to the iron's soleplate, allowing it to become hot and suitable for ironing clothes.

b) Electric immersion heater: An electric immersion heater also utilizes the heating effect of current. It consists of a heating element that is immersed in a liquid, such as water. When the electric current passes through the heating element, it produces heat, which is transferred to the liquid, resulting in heating the water.

c) Electric geyser: Similar to an electric immersion heater, an electric geyser also utilizes the heating effect of current. It has a heating element that heats the water in a storage tank. When the electric current passes through the heating element, it produces heat, which is transferred to the water, allowing it to reach the desired temperature for bathing or other purposes.

d) A battery: Unlike the previous options, a battery does not rely on the heating effect of current. A battery converts chemical energy into electrical energy. It consists of one or more electrochemical cells, which produce a potential difference (voltage) between the positive and negative terminals. This potential difference allows the flow of electric current in a circuit, but the primary purpose of a battery is not to produce heat.

**Conclusion:**

The correct answer is option D, a battery. While electric iron, electric immersion heater, and electric geyser utilize the heating effect of current to produce heat, a battery's primary function is to provide electrical energy for various devices and systems, rather than generating heat.