Q. Assertion (A) Half of the charge of an electron does not exist.
Reason (R) Electric charge is quantized.

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Solution:

The charge on any body can be expressed as the integral multiple of basic unit of charge i.e., charge on one electron. This phenomena is called quantisation of electric charge.
It can be written as , where
Charge is said to be quantised because it can have only discrete values rather than any arbitrary value, i.e., free particle has no charge at all or a charge of or but not a free particle with a charge of, say .
From above discussion, it is cleared that electric charge is quantised, which means charge can be written as integral multiple of charge an electron but it cannot be written as integral multiple of charge on half electron. It is because half of the charge of an electron does not exist.
Hence, assertion and reason both are correct and assertion is correct explanation.