The correct answer is $\boxed{\text{B}}$.
The resistance of a conductor is given by the equation $R = \frac{V}{I}$, where $V$ is the voltage across the conductor and $I$ is the current through the conductor. In this case, we are given that $V = 240 \text{ V}$ and $I = 300 \text{ mA} = 0.3 \text{ A}$. Substituting these values into the equation, we get $R = \frac{240 \text{ V}}{0.3 \text{ A}} = 800 \Omega$. Therefore, the resistance of the light bulb is 800 ohms.
Option A is incorrect because 400 ohms is not the resistance of the light bulb. Option C is incorrect because 800 ohms is not the resistance of the light bulb. Option D is incorrect because 1000 ohms is not the resistance of the light bulb.