Sugar is the solute since it is the thing that is being dissolved.
And the water is the solvent since it is the thing that contains the solute.
I believe the answer is a theory.
b is fastest, having the lowest activation energy (35 kJ) and is an exothermic reaction, releasing energy in the form of heat
c is slowest, having the highest activation energy (55 kJ) and is an endothermic reaction, taking in energy from its surroundings