Which of the following descriptions fits a Red Roan horse best?

The description that fits a Red Roan horse best is the one that indicates a uniform mixture of white with red hairs. Red Roan horses are characterized by their coat color, where red and white hairs are blended throughout the body, creating an even, balanced appearance. This distinct color pattern is what primarily defines a Red Roan, distinguishing it from other roan variations and horse colors.

In considering the other descriptions, a uniform mixture of white and black hairs corresponds to a Black Roan, which features a blend of black and white hairs rather than red. The mention of a Dun horse without a dorsal stripe describes a completely different color type that is typically yellowish or tan, with a distinct appearance due to the lack of darker or colored elements that would characterize a roan. Lastly, the description of a smoky or mouse-colored body aligns more closely with the colors associated with dilution or specific palomino or smoky black types, rather than the defined color pattern of a Red Roan.