What is necessary for light to propagate?

Light waves are electromagnetic waves, which do not need any medium to propagate. Light waves also travel in vacuum even. Other types of waves like sound waves does require a medium to travel. For sound waves a medium is mandatory. But for light waves no medium is necessary because light is consists of energy packets which Einstein called photons (particles), photons can travel through space in shape of a stream of small particles. They can even travel more frequently in space and can loss less energy because molecules are not present in their way to slow them down. Light is carried by those photons and also other forms of light like electromagnetic radiations like X-rays, microwaves and radio waves. Electromagnetic waves transfer its energy from one point to another is called propagation of light. Three main processes occur when light travels through boundaries of one medium to another namely, Transmission, Reflection, and Refraction.

d eras and periods

it has to be b, about 97 percent.