Assertion $(A)$: Atoms with completely filled and half-filled subshells are stable.
Reason $(R)$: Completely filled and half-filled subshells have symmetrical distribution of electrons and have maximum exchange energy.
The correct answer is:

• A
  $(A)$ and $(R)$ are correct, $(R)$ is the correct explanation of $(A)$
• B
  $(A)$ and $(R)$ are correct, $(R)$ is not the correct explanation of $(A)$
• C
  $(A)$ is correct, but $(R)$ is not correct
• D
  $(A)$ is not correct, but $(R)$ is correct