What is the basic cause of quantisation of charge?

Hint: If protons and electrons are the most fundamental particles and only charge carriers in the universe, then all the observable charges must be integral multiples of multiple electrons and protons.

Step by Step Answer
Charge quantization is the principle that the charge of an object is an integral multiple of the elementary charge.
Now, since protons and electrons are the only charge carriers in the universe, therefore all the observable charges must be integral multiple of electron. If an object contains $n$, electrons and ${n_2}$ protons, then the net charge on object is:
$ - {n_1}\left( e \right) + {n_2}\left( e \right) = \left( {{n_1} - {n_2}} \right)e$
Indeed, there are elementary particles other than protons and electrons, which carry charge. But all the elementary particles have charges which are integral multiple of $e$.. Thus charge on any object is always an integral multiple of $e$ and can be changed in steps of$e$, i.e. charge is quantized.

Note:
The step size $e$ is usually so small that we can easily neglect the quantization. If $l\,\mu \,C$ contains $n$ units of basic charge $e$ where,
$n = \dfrac{{l\,\mu \,C}}{{1.6 \times {{10}^{ - 19C}}}} = 6 \times {10^{12}}$
The step size is thus very small as compared to the charges usually found. Hence in many cases, we assume a continuous charge variation.