Final answer:
The power of the bulb will decrease by roughly 4% if the voltage across it is decreased by 2%, assuming that the resistance remains unchanged, because power is proportional to the square of the voltage.
Step-by-step explanation:
If we assume that the resistance of the bulb's filament remains unchanged, then the power of the bulb can be determined by the equation P = V²/R, where P is the power, V is the electrical potential difference (voltage), and R is the resistance.
A decrease of 2% in voltage means that the new voltage is 98% of the original voltage. Since power depends on the square of the voltage, if the original power is P, then the decreased power is (0.98V)²/R. This can be written as (0.98)² * (V²/R), which simplifies to approximately 0.9604 of the original power, or a decrease of 3.96%. So, the correct answer is a decrease of roughly 4%.