An igneous rock that cools rapidly is likely to have small crystals. The Option A is correct.
In geography, the rapid cooling of molten rock prevents the formation of large crystals because there is not enough time for the mineral constituents to grow and arrange themselves into a well-developed crystal lattice structure. But instead, the small crystals and glassy texture can result.
This occurs because the atoms in the molten rock are forced to freeze quickly, locking them into a disordered structure which is characteristic of rocks like obsidian.
However, rocks that cool slowly deep within the Earth's crust have more time for crystal growth resulting in larger crystals such as those found in granite. So, the speed of cooling is a key factor in determining the size of crystals in an igneous rock.