Question

The Time when the sun begins to cast its light in the sky is known as
[A] Dusk
[B] Dawn
[C] Sunrise
[D] Sunset

Answer 1

Hint :In order to solve this question we must remember that the Sun never sets or rises, dusk or dawn, sunrise and sunset are phenomena that occur due to rotation of earth on its own axis. Time is different that proves that events occur one after another.

Complete Step By Step Answer:
Let us examine each option given in the question
Considering option [A] Dusk
Dusk: Dusk is that time period which usually happens after the sun sets down completely and it is almost dark. Speaking astronomically it is that darkest stage of twilight before night begins. It is that moment when the sun is $18$ degree below the horizon In evening.
So according to question this is NOT correct
Considering option [B] Dawn
Dawn: Dawn is that time period which happens before sunrise or in other words phenomena that happens just before sunrise known as pre-sunrise. So in this time period the sun begins to cast its light in the sky. Speaking astronomically it is that moment when the sun is $18$ degree below the horizon in morning
So according to given question this option is correct
Considering option [C] Sunrise
Sunrise: Sunrise is that time period when the sun begins to rise from the horizon in the morning. It happened after dawn.
Red light prevails in sky because of its lower refractive index than others in VIBGYOR so it travels fast then other visible wavelengths
So according to question this is NOT correct
Considering option [D] Sunset
Sunset: Sunset is that time period when sun begins to just down below horizon in evening. It happens before dusk. Here also red color prevails because of the scattering effect.
So according to question this is NOT correct
So the correct option is [B] Dawn.

Note :
It should be remembered that, here defined dusk and dawn is astronomical dawn and dusk. There are two other types of dusk and dawn too that is Civil Dusk, Dawn and the other type is Nautical Dusk, Dawn. Although all three dawns have a safe definition of pre-sunrise but having different values of degree. Also Dawn and Dusk are marked as the starting and end point of astronomical twilight respectively.