Which of the following has the smallest crystal size?

The smallest crystal size among the listed options corresponds to cryptocrystalline aggregates. These aggregates consist of crystals that are too small to be seen with the naked eye or even with a standard light microscope. The crystal structure is present, but the individual crystals are on a microscopic scale, often measuring less than 1 micron in diameter.

Microcrystalline aggregates, while still comprising small crystals, can be viewed under a microscope and have a slightly larger crystal size than cryptocrystalline aggregates. Macroscopic crystals and visible crystals are characterized by their larger sizes, easily identifiable and observable with the naked eye.

Thus, cryptocrystalline aggregates represent the smallest crystal size, illustrating a unique category within crystalline structures where the crystallinity is present but not visually apparent due to the minuscule size of the individual crystals.