Answer: (b) when the sun is behind you.
Rainbows can only be seen in the sky when the sun is behind us since the rainbow reflects the sun’s light, and reflection does not occur on the same side as the object, but on the other side.
A rainbow is regarded as one of the most spectacular light shows observed on the earth. A rainbow is a multicoloured arc made due to the striking of light on water droplets. Rainbow is produced after the rain, by reflection, refraction and light dispersion process in droplets of water. All such events develop a light spectrum in the sky are called rainbow.