Time period of simple pendulum is given by formula
[tex]T = 2\pi \sqrt{\frac{L}{g}}[/tex]
here
L = length of pendulum
g = acceleration due to gravity
now as per formula we can see that time period is directly proportional to the square root of the length.
So in order to increase the time period we can increase its length
so correct answer would be
b. by making the string longer
