Which type of brush is narrow, firm, and has a flat edge, used for applying makeup?

The correct choice is the concealer brush, which is designed to be narrow, firm, and feature a flat edge. This design allows for precise application of concealer to cover blemishes, dark circles, and other imperfections on the skin. The firmness of the bristles ensures that the product can be applied evenly and effectively, while the narrow shape helps to reach smaller areas around the eyes and nose. The flat edge provides a smooth finish, allowing for specific application techniques without spreading the product beyond the desired area.

In contrast, a flat brush typically has a broader surface and may not be as suitable for detailed work. A blending brush is generally fluffy and meant for diffusing colors rather than applying product, and an angled brush is usually used for contouring or applying eyeliner, rather than for the precise application of concealer. Thus, the unique characteristics of the concealer brush make it the best choice for the specified application.