The correct answer is: D. Unity.
The power factor of an electric bulb is unity because it is a resistive load. Resistive loads consume real power, which is the product of the voltage and current. The voltage and current are in phase in a resistive load, so the power factor is unity.
A lagging power factor occurs when the current lags the voltage. This can happen in inductive loads, such as motors. Inductive loads store energy in a magnetic field, and this energy causes the current to lag the voltage.
A leading power factor occurs when the current leads the voltage. This can happen in capacitive loads, such as capacitors. Capacitive loads store energy in an electric field, and this energy causes the current to lead the voltage.
In general, a power factor of unity is desirable because it means that the load is consuming real power. A lagging or leading power factor can cause problems in power systems, so it is important to correct these power factors.