The correct answer is C. Its length should be quadrupled.
The period of a simple pendulum is given by the formula $T = 2\pi\sqrt{\frac{L}{g}}$, where $L$ is the length of the pendulum and $g$ is the acceleration due to gravity. We can see from this formula that the period is proportional to the square root of the length of the pendulum. Therefore, if we want to double the period, we need to quadruple the length of the pendulum.
The mass of the bob does not affect the period of the pendulum. This is because the force of gravity on the bob is proportional to its mass, and the acceleration of the bob is also proportional to its mass. Therefore, the net force on the bob is independent of its mass, and the period of the pendulum is also independent of its mass.
I hope this explanation is helpful. Please let me know if you have any other questions.