What is the international system of units? What are the supplementary quantities?

The unit of energy is:

The total plane angle subtended by a circle at its centre is ......... $rad$.

Define density and give the $MKS$ and $CGS$ units of density.

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.

$A$ physical quantity is measured and its value is found to be $nu$, where $n$ is the numerical value and $u$ is the unit. Then which of the following relations is true?