The correct answer is A. Must be decreased.
A second’s pendulum is a simple pendulum that swings from its rest position to its maximum displacement and back again in exactly two seconds. The period of a 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.
If a second’s pendulum gains 2 minutes a day, then it is swinging too quickly. This means that the length of the pendulum is too long. To make the pendulum swing more slowly, we need to decrease its length. This will cause the period of the pendulum to increase, and the pendulum will therefore gain less time each day.
Option B is incorrect because increasing the length of the pendulum will cause it to swing more quickly, which will make it gain more time each day. Option C is incorrect because the weight of the bob does not affect the period of the pendulum. Option D is incorrect because decreasing the weight of the bob will also cause the pendulum to swing more quickly.