Correct choice is (a) 24 x (3/5)^1/2 hr
Easiest explanation: The time period of a satellite is given by;
T = 2 x pi x (R+h)^3/2 / (G x M)^1/2
For a geostationary satellite;
24 = (5R)^3/2 x (constant); [As per giver information]
Required;
t = (3R)^2/2 x (constant)
t = Time period of the satellite at a height of 2.5R
t/24 = (3R/5R)^3/2
= (3/5)^3/2
t = 24 x (3/5)^1/2 hr.