The correct statement is:

Solution & Explanation

Concept: maximum covalency = number of valence orbitals available for bonding. Boron (2nd period, Z=5) has only the four orbitals 2s, 2p_x, 2p_y, 2p_z in its valence shell. There is no 2d subshell, so it can form at most four bonds — its maximum covalency is 4 (seen in BH₄⁻ and BF₄⁻, formed when a lone pair completes the octet). This makes option A correct. Checking the others: - Beryllium (2nd period) also uses 2s + three 2p orbitals = four valence orbitals, not three. So B is wrong. - Magnesium (3rd period) has accessible 3d orbitals and can show covalency up to six, not four. So C is wrong. - Aluminium (3rd period) has 3s, three 3p and five 3d = nine valence orbitals, not five. So D is wrong. Trap: do not confuse the actual covalency boron usually shows (3) with its maximum possible covalency (4). Hence the answer is A — Boron has a maximum covalency of four.