The correct answer is $\boxed{\text{B. }\frac{{\text{E}}}{2}}$.
The energy of a simple pendulum is given by the equation $E = \frac{1}{2}m\omega^2L^2$, where $m$ is the mass of the pendulum, $\omega$ is the angular frequency, and $L$ is the length of the pendulum. The angular frequency is given by the equation $\omega = \sqrt{\frac{g}{L}}$, where $g$ is the acceleration due to gravity.
If the length of the pendulum is doubled, the angular frequency will be halved. This is because the angular frequency is inversely proportional to the square root of the length of the pendulum. The energy of the pendulum is proportional to the square of the angular frequency, so the energy of the pendulum will be halved when the length of the pendulum is doubled.
Option A is incorrect because the energy of the pendulum is not constant. The energy of the pendulum depends on the length of the pendulum.
Option C is incorrect because the energy of the pendulum is not doubled when the length of the pendulum is doubled. The energy of the pendulum is halved when the length of the pendulum is doubled.
Option D is incorrect because the energy of the pendulum is not quadrupled when the length of the pendulum is doubled. The energy of the pendulum is halved when the length of the pendulum is doubled.