Assertion: Be and Al have similar properties.
Reason: Be2+ and Al3+ have nearly the same charge/volume ratios and similar polarising power.
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 ( Li+ = 0.76 A,Mg2+ = 0.72)
This is not true for Be2+ (=0.45A) and Al3+ (=0.535 A). Here the charge per unit area (or charge/volume) ratios are similar (respectively 2.36 and 2.5)