Step 1: Using the formula for the time period.
The time period \( T \) 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.
Step 2: Effect of changing the length.
If the length is increased by a factor of 4, the time period increases by a factor of \( \sqrt{4} = 2 \). Hence, the time period will be doubled.
Step 3: Conclusion.
The correct answer is (D), increase the length four times.