Assertion: $\mathrm{Co}(\mathrm{IV})$ is known but $\mathrm{Ni}(\mathrm{IV})$ is not. Reason : $\mathrm{Ni}(\mathrm{IV})$ has $d^4$ electronic configuration.

• 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.
• C

  If assertion is true but reason is false.
• Correct answer
  D

  If both assertion and reason are false.

Solution

Both Co and Ni have (IV) oxidation state. $\mathrm{Ni}$ (IV) has $3 d^6$ electronic configuration.

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