The period of a simple pendulum, whose bob is a hollow metallic sphere is T. The period is T1 when the bob is filled with sand, T2 when it is filled with mercury and T3when it is half filled with mercury. Which of the following is true?

Time period of pendulum doesn't depends upon mass but it depends upon length (distance between point of suspension and centre of mass). In first three cases length are same so T = T1 = T2 but in last case centre of mass lowers which in tum increases the length. So in this case time period will be more than the other cases.