What is $1$ radian? What is $1$ steradian?

$second^{-1}$ is the unit of:

Explain this statement clearly: "To call a dimensional quantity 'large' or 'small' is meaningless without specifying a standard for comparison". In view of this,reframe the following statements wherever necessary:
$(a)$ atoms are very small objects
$(b)$ a jet plane moves with great speed
$(c)$ the mass of Jupiter is very large
$(d)$ the air inside this room contains a large number of molecules
$(e)$ a proton is much more massive than an electron
$(f)$ the speed of sound is much smaller than the speed of light.

Explain the difference between $nm$, $mN$, and $Nm$.

The $SI$ unit of length is 'metre'. Suppose we adopt a new unit of length which equals $x$ metre. Then,the area of $1 \ m^2$ expressed in terms of the new unit has a magnitude of:

Which of the following is not a physical quantity?