Answer: A (aluminium). In metallic bonding each atom releases its valence (outer-shell) electrons into the delocalised 'sea of electrons'. So per mole of metal, the element with the most valence electrons per atom contributes the most electrons to the sea.
Aluminium is in Group III, giving 3 valence electrons per atom, so one mole of aluminium contributes 3 moles of electrons. Barium and strontium are in Group II with 2 valence electrons each, contributing 2 moles. Caesium is in Group I with just 1 valence electron, contributing 1 mole.
Aluminium therefore puts the most electrons into the sea, so the answer is A. A common error is to pick the heaviest atom, but the number of electrons in the sea depends only on the number of outer-shell electrons donated, not on the mass or size of the atom.
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