Assertion (A): $\mathrm{K}, \mathrm{Rb}$ and $\mathrm{Cs}$ form superoxides
Reason (R): The stability of superoxides increases form $\mathrm{K}$ to $\mathrm{Cs}$ due to decrease in lattice energy.

• A

  Both $\mathrm{A}$ and $\mathrm{R}$ are true and $\mathrm{R}$ is a correct explanation for $\mathrm{A}$
• B

  Both $\mathrm{A}$ and $\mathrm{R}$ are true but $\mathrm{R}$ is not a correct explanation for $\mathrm{A}$
• Correct answer
  C

  $\mathrm{A}$ is true, $\mathrm{R}$ is false
• D

  $\mathrm{A}$ is false, $\mathrm{R}$ is true

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