Which of the following metals has the highest melting point?
a: sodium
b: magnesium
c: aluminium
d: potassium

The metal with the highest melting point is aluminium which melts at 660.3°C.

Magnesium melts at 639°C , sodium at 98°C and potassium at 64°C.

The melting point is the temperature at which a substance  begins to melt. As a solid substance is heated, or absorbs heat from the environment,  the molecules that make up that substance begin to gain more (heat) energy, enough to overcome the intermolecular forces that in the first place have been holding them together in a sort of rigid fashion.

This energy enables the molecules  to break free of their intermolecular forces and thus the solid's lattice structure changes and the solid becomes liquid.