$S$ - Statement: The amino acid with a non-polar $R$ group is serine.
$R$ - Reason: The amino acid with a polar and uncharged $R$ group is threonine.

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