What is meant by air pressure?
a. the force of air (f)
b. the force of air per distance (f/d)
c. the force of area per area (f/a)
d. the force of air per volume (f/v)

The pressure exerted on a surface is by definition the ratio of the force acting on the surface divided by the area of the surface:

P = F/A

P = pressure, F = force, A = area

Choice C

speed of light in vacuum is 300,000 km/s

speed of light in other mediums is given by the expression v = c/n, where c is the speed of light in vacuum and n is the refractive index of medium.

for the first medium v = 200,000 km/s

  so, refractive index of medium = 300000/200000 = 1.5

    1.5 refractive index is usually for glass. so first medium is glass.

for the second medium v = 250,00 km/s

so, refractive index of medium = 300000/250000 = 1.2

    1.2 refractive index is usually for water. so second medium is water.

for the third medium v = 0 km/s

so, refractive index of medium = 300000/0 = ∞

    ∞ refractive index is usually for opaque mediums. so third medium is opaque medium.