Which type of nut is designed to be secured with cotter pins or safety wire?

The correct answer identifies the type of nut designed specifically to be secured with cotter pins or safety wire as Castle Nuts. Castle nuts are recognizable by their slotted top which allows for the insertion of cotter pins or safety wire to prevent loosening due to vibration or axial movement. This design ensures that once the nut is tightened to the appropriate torque, it remains secure under operational conditions.

In contrast, plain hex nuts do not have any provisions for securing such as slots or holes, meaning they rely solely on friction and proper torque to stay in place. Plate nuts, designed to attach to a flat surface, also lack the features necessary for additional security with cotter pins or safety wire. Similarly, check nuts are typically used in conjunction with another nut for locking purposes but do not incorporate the specific features that allow for secure placement with cotter pins or safety wire. Thus, castle nuts are uniquely suited for applications where extra assurance against loosening is necessary.