$STATEMENT-1$: Boron always forms covalent bonds.
Because
$STATEMENT-2$: The small size of $B^{3+}$ favors the formation of covalent bonds.

• A
  $Statement-1$ is True,$Statement-2$ is True; $Statement-2$ is a correct explanation for $Statement-1$.
• B
  $Statement-1$ is True,$Statement-2$ is True; $Statement-2$ is $NOT$ a correct explanation for $Statement-1$.
• C
  $Statement-1$ is True,$Statement-2$ is False.
• D
  $Statement-1$ is False,$Statement-2$ is True.