Assertion : $\mathrm{O}_2$ is paramagnetic.Reason : It has one unpaired electron.

• A

  If both assertion and reason are true and reason is the correct explanation of assertion
• B

  If both assertion and reason are true but reason is not the correct explanation of assertion
• Correct answer
  C

  If assertion is true but reason is false
• D

  If both assertion and reason are false.

Hence, the correct answer is option: 'C'.