Which of the following materials is lighter than steel and commonly used in aerospace?

The use of carbon fiber in aerospace applications is primarily due to its advantageous strength-to-weight ratio, which is significantly greater than that of steel. Carbon fiber reinforced composites are not only lighter but also provide excellent stiffness and durability, making them ideal for aircraft structural components. The lightweight nature of carbon fiber helps improve fuel efficiency and performance, as aircraft require materials that minimize weight while providing the necessary strength to withstand operational stresses.

In contrast, while wood is a natural material that can also be lightweight, it is not commonly used in modern aerospace designs due to its susceptibility to environmental factors and lower strength compared to composite materials. Concrete, although strong, is much heavier and used in different contexts, such as airport runways but not in airframe construction. Brass is a dense metal alloy often used in fittings and fasteners, but its weight is not suitable for widespread application in the aerospace industry where maintaining a low weight is crucial. Therefore, carbon fiber stands out as the material that effectively meets the demands of aerospace engineering.