The phenomena involved in the reflection of radiowaves by ionosphere is similar to

1. reflection of light by a plane mirror
2. total internal reflection of light in the air during a mirage
3. dispersion of light by  water molecules during the formation of a rainbow
4. scattering of light by the particles of air

(2) Hint: The radiowaves are total internally reflected by the ionosphere.

The phenomenon involved in the reflection of radiowaves by ionosphere is similar to total internal reflection of light in the air during a mirage i.e., angle of incidence is greater than the critical angle.