Which choice is NOT a basic horse coat color?

In the context of horse coat colors, gray is not considered a basic base color like the others listed. Basic horse coat colors typically include black, brown, and chestnut, which are defined by their genetic makeup and pigmentation.

Black refers to horses with a solid black coat, brown can vary from light to dark shades but typically has a brownish hue, and chestnut is recognized by its reddish-brown color. Gray, on the other hand, is a color that results from the graying gene which causes horses to lose their original coat color as they age, leading to variations from gray to white. As such, while gray is a common coat color seen in horses, it is not one of the foundational or basic coat colors from which other colors can derive, which is why it is identified as the correct answer to the question.