Which of the following materials is commonly used as a mechanical fastener?

Steel bolts are commonly used as mechanical fasteners because they provide strong, reliable connections between components in a wide variety of applications. They are made from steel, which offers high tensile strength and durability, making them ideal for securing parts in structural, automotive, and machinery settings. The threaded design of bolts allows them to be tightened securely, and they can be easily installed and removed as needed, providing flexibility in assembly and maintenance.

In contrast, aluminum oxide is primarily known for its applications as an abrasive and as a component in ceramics, rather than as a fastener. Weld filler wire is specifically used in welding applications to join metals, not as a standalone fastening mechanism. Plastic sheets, while versatile in many applications, do not provide the necessary strength and integrity typically required for mechanical fastening. Thus, the use of steel bolts stands out as the clear choice for mechanical fastening needs.