What rock texture features fine grains that are not visible to the naked eye?

The appropriate answer highlights that the texture described is characterized by fine grains that are too small to be seen without the aid of a microscope. Aphanitic texture occurs in igneous rocks that cool quickly, leading to the formation of very small mineral crystals that are often indistinguishable to the naked eye. This rapid cooling typically happens when magma is erupted onto the surface, allowing it to solidify quickly, resulting in a smooth or fine-grained appearance.

In contrast, phaneritic texture is associated with igneous rocks that have cooled slowly underground, allowing larger crystals to form that are visible without magnification. Granular texture refers to a general term that may not specifically indicate the lack of visibility of grains and can apply to various grain sizes. Coarse-grained texture, as the term suggests, pertains to rocks with larger, easily discernible grains, which is the opposite of what is defined in the question.

Thus, the focus on the invisibility of fine grains to the naked eye makes aphanitic texture the correct choice, as it directly reflects the characteristics described.