The correct answer is A.
Entropy is a measurement of disorder, and it increases when a system
becomes more random. When a solid dissolves, the highly ordered crystal structure
is replaced by the relatively disordered solution phase. As an example, take
the MgCl2 of the previous question: in the solid state,
the magnesium and chloride ions alternate in the lattice because of their
opposing charges. The ions are fixed rather rigidly in place throughout the
solid, and vibrate about some relatively small equilibrium distance from each
other. The fixed nature of their relative positions and distances from one
another results in a high level of three dimensional order throughout the
crystal. Additionally, the relatively high density of the solid implies that
the mass of solid will occupy a relatively small volume of space. When the
MgCl2 is dissolved in water, the ions separate and
are now free to move about the entire solution; the particles thus experience
an increased freedom of motion in an increased volume of space. The result
is that with more possibilities for location, speed, and direction, there
is a net increase in the randomness of their motion and entropy is thus increased
as well.