Why Be and Al are compared
Beryllium and aluminium show a diagonal relationship, so they have several similar properties.

Check each option

Option B: Amphoteric nature of oxides

• Beryllium oxide: $\mathrm{BeO}$ is amphoteric.
• Aluminium oxide: $\mathrm{Al_2O_3}$ is also amphoteric.
• So this is a similarity, not a difference.

Option C: Forming covalent halides

• $\mathrm{BeCl_2}$ is covalent.
• $\mathrm{AlCl_3}$ is also largely covalent and exists as dimer $\mathrm{Al_2Cl_6}$.
• So this is also a similarity.

Option D: Forming polymeric hydrides

• Beryllium forms polymeric hydride: $\mathrm{BeH_2}$.
• Aluminium forms polymeric hydride: $\mathrm{AlH_3}$.
• Again, this is a similarity.

Option A: Exhibiting maximum covalency in compound

• Beryllium has valence shell configuration $2s^2$ and no available d-orbitals, so its maximum covalency is limited to 4.
• Aluminium can show higher coordination/covalency in species like $\mathrm{[AlF_6]^{3-}}$, so its maximum covalency can be 6.
• Therefore, this is a difference between Be and Al.

Conclusion The property in which beryllium and aluminium differ is: $\boxed{\text{A: exhibiting maximum covalency in compound}}$

Comparison with stored answer

• Derived answer: A
• Stored correct answer: A
• They agree.