One quantum of charge should be at least be equal to the charge in coulomb:

Hint: In physics, quantum means the minimum possible value of an entity. Then it said that the magnitude of that entity can only be an integer multiple of one quantum. One quantum of charge is equal to the magnitude of charge on an electron.

Complete answer:
Let us first understand what is meant by the word quantum.
In physics, quantum means the minimum possible value of an entity. The plural of quantum is quanta. Then it said that the magnitude of that entity can only be an integer multiple of one quantum.
Consider a pack of balls. Let us say that here the magnitude is the total number of balls in the pack. The minimum number of balls in the pack is one (the pack has at one ball for sure). The ball is one quantum. Then the total number of balls in the pack will only be an integer multiple of this quantum. It cannot be any other value.
Similarly, the quantity charge is also quantized. The value of one quantum is equal to the magnitude of the charge on an electron. We know that the magnitude of charge on an electron in coulomb is $1.6\times {{10}^{-19}}C$.

Hence, one quantum of charge has a charge of $1.6\times {{10}^{-19}}C$ and one quantum of charge is denoted as ‘e’.

Note:
Let us discuss why an electron is chosen as the one quantum. This simply because electrons and protons are the fundamental charged particles and also the charges are made up of some whole numbers of electrons. That is a charge on a body is an integer multiple of e.