Assertion $(A)$: $Mg^{2+}$ and $Al^{3+}$ are isoelectronic but the magnitude of the ionic radius of $Al^{3+}$ is less than that of $Mg^{2+}$.
Reason $(R)$: The effective nuclear charge on the outermost electrons in $Al^{3+}$ is greater than that in $Mg^{2+}$.
The correct option among the following is:

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