Which component is essential for securing bolts to prevent them from loosening under stress?

The nut is essential for securing bolts because it is designed to fit tightly against the bolt's threads, creating a strong fastening mechanism. When a bolt is inserted through materials and paired with a nut, the nut is tightened onto the bolt, which generates a clamping force that keeps the materials securely in place. This clamping force helps to resist loosening due to vibrations or dynamic loads. In a typical assembly, as the nut is turned, it moves along the threads of the bolt, which pulls the bolt into the material and enhances stability.

The other choices, while related to fastening, do not provide the same locking functionality in the context of securing bolts. A bolt itself is a fastener but requires a nut or other securing method to hold it in place. A washer can provide a larger bearing surface and can help distribute load but does not secure the assembly on its own. A screw, while also a fastener, is typically designed to cut its own threads into the material and does not operate in conjunction with a simple bolt and nut configuration. Thus, the nut plays a critical role in ensuring that the bolt remains securely fastened.