Step 1: Understanding the Concept:
Quantisation of electric charge means that charge exists in discrete packets. The smallest basic unit of charge that can be transferred during charging is the magnitude of charge on an electron ($e = 1.6 \times 10^{-19}$ C).
Step 2: Detailed Explanation:
When an object becomes charged, it is because electrons have been added to it or removed from it.
Since electrons are fundamental particles that cannot be split into smaller pieces during ordinary physical processes, they must be transferred in whole numbers (integers).
Therefore, the total charge \( Q \) on any body is always an integral multiple of the elementary charge \( e \), expressed as \( Q = \pm ne \).
The experimental fact that only integral numbers of electrons are transferred is the primary evidence for the quantisation of charge.
Step 3: Final Answer:
The blank should be filled with "electrons".