What type of candy relies heavily on sugar crystallization?

Rock candy is primarily made through the process of sugar crystallization, which is central to its formation. In making rock candy, a supersaturated sugar solution is allowed to cool slowly, during which sugar molecules begin to crystallize out of the solution and attach themselves to a stick or string. The larger the crystals grow, the more pronounced the final candy's crystalline structure becomes, resulting in the characteristic appearance and texture of rock candy.

In contrast, marshmallows, toffee, and gummy candies involve different processes. Marshmallows rely on whipping air into a sugar syrup and stabilizing it with gelatin, which does not involve significant crystallization. Toffee is made by cooking sugar to a high temperature, leading to caramelization rather than crystallization. Gummy candies derive their texture from the use of gelatin and sugar, creating a chewy consistency without the crystal structures seen in rock candy. Therefore, the distinctive reliance on crystallization makes rock candy the correct answer in this context.