A board of fine, upright nails that is used to comb or detangle human hair extensions or to blend colors is a:

The correct answer is that a board of fine, upright nails used to comb or detangle human hair extensions or to blend colors is known as a hackle. The hackle's design allows for efficient separation and blending of hair fibers, making it an essential tool for professionals working with hair extensions.

It helps to eliminate tangles, ensuring the hair remains smooth and manageable. Additionally, the upright nails facilitate mixing different shades or textures of hair, enabling stylists to create a seamless and natural appearance when applying extensions.

This tool is particularly valued in the hair extension industry for its effectiveness in preparing hair for weaves and braids, emphasizing the importance of proper tools in achieving quality results. The other options do not serve this specific purpose. For example, a drawing board is typically used for sketching or planning, while a leather board might be used for a different type of craft or manipulation of materials. An extension broadly refers to the added hair itself rather than the tool used for styling or preparing it.