I'm pretty sure it's D. Compounds
Salt water is an example of a solution, and a solution is a type of mixture, so by multiple choice logic none can be the right answer! Seriously though - mixture are combinations of two compounds that are physically combined but do not undergo a chemical reaction with one another to produce a new compound. For instance, when you dissolve salt in water, you don't produce any new products, and you can boil away the water to leave only the salt (a physical method of separation). You cannot separate a compound by physical means because by definition this would be a chemical reaction as you're breaking bonds.
Band theory
The atomic orbitals of metals combine to form <em>molecular orbitals</em>. The molecular orbitals are so close in energy that they form a <em>band</em> of energies.
<em>Ionic bond theory</em> is incorrect because there are no ions in metals.
<em>Covalent bond theory</em> is incorrect because there are no covalent bonds in metals.
<em>Electron bond theory</em> is incorrect because there is no theory with that name (but there is a <em>valence bond theory</em>).
