Diffraction enables longer waves to bend around obstacles

Steps:

• Compare wavelengths: radio waves (15 km) are much longer than microwaves (1.5 cm).
• The mountain blocks the direct line-of-sight path between transmitter and receiver.
• Longer-wavelength radio waves diffract significantly around the mountain.
• Shorter-wavelength microwaves diffract minimally and cannot reach the receiver.

Why B is correct:

• Diffraction spreads waves around obstacles when wavelength is comparable to or larger than obstacle dimensions; radio waves' long wavelength allows bending around the mountain, unlike microwaves.

Why the others are wrong:

• A: Both waves from the aerial are coherent electromagnetic waves.
• C: All electromagnetic waves, including radio and microwaves, travel at the speed of light in vacuum.

Final answer: B