Answer: A (giant ionic lattice). A high melting point means a giant structure. Conducting when molten but not when solid is the signature of an ionic compound: the ions are locked in place in the solid lattice but become free to move when the lattice breaks down on melting.
A metallic structure would conduct as both a solid and a liquid (the electrons are always mobile). A giant covalent structure generally does not conduct at all. A simple molecular substance has a low melting point. Only a giant ionic lattice fits every clue. Answer A.
Every O Level Pure Chemistry TYS MCQ solution video, plus full Sec 3 / Sec 4 topic recordings.
Find out more