The transfer of integral number of _______is one of the evidence of quantization of electric charge.

The correct option is electrons.

Explanation:
Quantization of electric charge is the principle that the electric charge of any system is not continuous, but instead exists in discrete packets that are integral multiples of a basic, fundamental unit of charge. This fundamental unit is the magnitude of the charge carried by a single electron.

When physical processes like friction, conduction, or induction cause an object to become charged, this charge transfer occurs through the movement of electrons from one body to another. Because electrons are stable, indivisible elementary particles in these processes, they can only be transferred in whole numbers.

Consequently, the net charge acquired by any body must be an integral multiple of the elementary charge. This relationship can be mathematically expressed as:

$$q=\pm ne$$

where:
• $q$ represents the total net charge,
• $n$ is a positive integer (1, 2, 3, ...), and
• $e$ is the elementary charge of an electron, which is approximately 1.6 × 10^-19 Coulombs.

Since we cannot transfer a fraction of an electron, the experimental observation that charge is always transferred in integral steps of electrons is the primary evidence for the quantization of electric charge.