Assertion: Be and Al have similar properties.

Reason: $B{e}^{2+}$ and $A{l}^{3+}$ have nearly the same charge/volume ratios and similar polarising power.

The correct option is A

Both the assertion and the reason are correct. The reason is the correct explanation for the assertion

Usually the diagonal relationships between $Li$ and $Mg$ is attributed to their similar sizes ( $L{i}^{+}=0.76A,M{g}^{2+}=0.72$)

This is not true for $B{e}^{2+}(=0.45A)$ and $A{l}^{3+}(=0.535A$). Here the charge per unit area (or charge/volume) ratios are similar (respectively 2.36 and 2.5)