As the rate of cooling of igneous rocks increases, the size of the crystals that form

The correct answer is "decreases".

Explanation:

As the rate of cooling of igneous rocks increases, the size of the crystals that form decreases. Volcanic or igneous rocks go trough a cooling process when they reach the Earth's surface, crystallizing and forming different crystal forms. The size of the crystals depend on the cooling rate, slow cooling allow the formation of large crystals, while fast cooling yields small crystals.