15) D

Let us start by calculating the time delay first.

Total distance = 45000 + 45000 = 90000 Km

Speed = 3.0 ×108 m/s = 3.0 ×10km/s

Time = (90000) ÷ (3.0 ×10km/s) = 0.3 s

We can rule out options A, B, C, E, F and G. We are left with options D and H.

We know the formula = 

c = ƒ x λ 

λ = c/ƒ = [(3.0 × 108) ÷(1.5 × 1010 Hz)] = 0.02 m 

Since the number obtained for wavelength is larger than wavelength of the visible light, the waves must be microwaves.

Hence the answer is: D