Beryllium and aluminium exhibit many properties which are similar. But the two elements differ in

Beryllium ($$\text{Be}$$, atomic number 4) and Aluminium ($$\text{Al}$$, atomic number 13) share a diagonal relationship in the periodic table, meaning they exhibit very similar chemical and physical properties due to their comparable ionic sizes and charge-to-size ratios

in their maximum covalency due to the availability of electron orbitals:

• Beryllium ($$\text{Be}$$): Belongs to the 2nd period. Its electronic configuration is $$1s^2 2s^2$$. Since it only has $$2s$$ and $$2p$$ orbitals available in its valence shell, it can accommodate a maximum of 4 pairs of electrons. Therefore, its maximum covalency is 4 (e.g., in $$[\text{BeF}_4]^{2-}$$).
  
  
• Aluminium ($$\text{Al}$$): Belongs to the 3rd period. Its electronic configuration is $$1s^2 2s^2 2p^6 3s^2 3p^1$$. It has vacant $$3d$$ orbitals available in its valence shell, allowing it to expand its octet. Therefore, its maximum covalency is 6 (e.g., in $$[\text{AlF}_6]^{3-}$$).

Hence, Option A is correct