A: Hybridization of carbon is $\mathrm{s}{\mathrm{p}}^2$ in all its crystalline allotropes.

R: There are alternate double-single bonds in each allotrope of carbon.

• A

  Both Assertion & Reason are true and the reason is the correct explanation of the assertion.
• B

  Both Assertion & Reason are true but the reason is not the correct explanation of the assertion.
• C

  Assertion is true statement but Reason is false.
• Correct answer
  D

  Both Assertion and Reason are false statements.

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