What is the main use of an LDR?

An LDR, or Light Dependent Resistor, is primarily used to detect the presence of light. This component changes its resistance based on the intensity of light falling on it—lowering its resistance in brighter light and increasing it in dimmer conditions. This property allows the LDR to act as a sensor in various applications, such as automatic lighting systems, where it can turn lights on or off depending on ambient light levels, or in devices like cameras to adjust exposure automatically.

The other choices reflect functionalities that are not inherent to LDRs. Varying voltage in a circuit typically involves components like resistors or variable resistors (potentiometers), while storing electrical energy is primarily the role of capacitors or batteries. Controlling circuit current might involve different types of sensors or switches that are not specifically LDRs. Thus, the main and correct use of an LDR resides in its ability to detect light levels effectively.