Aluminium ($Z=13$) has the electronic configuration $[Ne] 3s^2 3p^1$. It has four valence orbitals ($3s, 3p_x, 3p_y, 3p_z$). Thus, the statement that it has five valence orbitals is incorrect.
Boron ($Z=5$) has the configuration $1s^2 2s^2 2p^1$. It has only four valence orbitals ($2s, 2p_x, 2p_y, 2p_z$). Due to the absence of d-orbitals, it cannot expand its octet beyond 8 electrons, limiting its maximum covalency to 4 (e.g., in $[BF_4]^-$). This statement is correct.
Beryllium ($Z=4$) has the configuration $1s^2 2s^2$. It has four valence orbitals ($2s, 2p_x, 2p_y, 2p_z$). The statement claiming it has three valence orbitals is incorrect.
Magnesium ($Z=12$) has the configuration $[Ne] 3s^2$. It has nine valence orbitals ($3s, 3p_x, 3p_y, 3p_z, 3d_{xy}, 3d_{yz}, 3d_{zx}, 3d_{x^2-y^2}, 3d_{z^2}$). Because it has vacant d-orbitals, its covalency can exceed four. Thus, the statement is incorrect.
Final Answer: (B)
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