Assertion: Diamagnetic substances are weakly repelled by external magnetic field.

Reason: All electrons are paired.

Assertion and Reason both are correct statements and reason is the correct explanation for assertion.

Diamagnetism

Diamagnetic substances are weakly repelled by a magnetic field. H₂O, NaCl and C₆H₆ are some examples of such substances.

They are weakly magnetised in a magnetic field in opposite direction.

Diamagnetism is shown by those substances in which all the electrons are paired and there are no unpaired electrons. Pairing of electrons cancels their magnetic moments and they lose their magnetic character.