An active component in a circuit is one that requires a power source to function and has the ability to control or amplify the electrical signal passing through it.

Option 1: A resistor is a passive component as it does not require a power source and does not have the ability to amplify or control the signal.

Option 2: A transistor is the correct answer as it is an active component. It requires a power source to function and has the ability to control and amplify the electrical signal. Transistors are widely used in electronic circuits for amplification and switching purposes.

Option 3: An inductor is a passive component that stores energy in a magnetic field. It does not require a power source to function and does not control or amplify the signal.

Option 4: A capacitor is a passive component that stores energy in an electric field. It does not require a power source to function and does not control or amplify the signal.

Therefore, the correct answer is option 2 as a transistor is an active component in a circuit.