What circuit component must be connected in series to protect an LED?

The component that must be connected in series to protect an LED is a series resistor. LEDs are current-sensitive devices, meaning they have a limited amount of current that they can handle before becoming damaged. The purpose of the series resistor is to limit the amount of current flowing through the LED, ensuring it operates within its safe parameters and avoiding excess heat that could cause failure.

When an LED is connected directly to a power source without a current-limiting resistor, it will draw too much current, potentially leading to immediate failure or a significantly reduced lifespan. By including a series resistor, you can calculate the appropriate value to control the current based on the power source voltage and the forward voltage drop of the LED, thereby providing necessary protection.

Other components listed do not serve this specific purpose in the context of protecting an LED. Inductors and transformers are related to managing voltage and current in alternating current circuits and would not be suitable for simple LED protection. Capacitors are used for filtering and energy storage, which also does not contribute directly to current limitation for an LED.