Assertion (A) ${\mathrm{Na}}^{+}\text{and }{\mathrm{Al}}^{3+}$ are isoelectronic.
Reason (R) The nuclear attraction on the outermost shell electrons of ${\mathrm{Al}}^{3+}$ is greater than that of ${\mathrm{Na}}^{+}$.

${\mathrm{Na}}^{+}\text{and }{\mathrm{Al}}^{3+}$ have 10 electrons, so, these are called isoelectronic species and the effective nuclear charge (ENC) value of ${\mathrm{Al}}^{3+}$ is greater than ENC value of ${\mathrm{Na}}^{+}$ due to larger positive charge in ${\mathrm{Al}}^{3+}$.

 + ve charge $\propto$ ENC