Answer:
a. shortening the string.
Step-by-step explanation:
Time period of a pendulum is given by
where,
L = Length of pendulum
g = Acceleration due to gravity = 9.81 m/s²
Here, the pendulum is slowing down this means that the time period is increasing. As the time period is proportional to the length it implies that the length of the pendulum is increasing.
Hence, in order to make the pendulum speed up or reduce the time period the length of the pendulum needs to be shortened.