The correct answer is
The 100 W bulb uses 2.5 times more energy
In fact, the power is the amount of energy used per unit of time. This means that the light bulb of 100 W uses 100 J in one second, and the light bulb of 40 W uses 40 J in one second, and if we compare the two numbers, we get
[tex] \frac{100 J}{40 J}=2.5 [/tex]
so, the 100 W light bulb uses 2.5 times more energy than the 40 W light bulb.
