$A$: No two electrons in an atom can have the same set of all four quantum numbers.
$R$: Two electrons in an atom can exist in the same shell,subshell,or orbital only if they have opposite spins.

• A
  $A$ and $R$ are both true and $R$ is the correct explanation of $A$.
• B
  $A$ and $R$ are both true,but $R$ is not the correct explanation of $A$.
• C
  $A$ is true,but $R$ is false.
• D
  $A$ is false,but $R$ is true.