What is a cotter pin primarily used for in mechanical applications?

A cotter pin is primarily used to secure the position of components by being inserted into a hole and then bent to hold it in place, making option C the most accurate choice. This method helps to prevent parts from coming undone from their intended position, which is crucial in maintaining the integrity of various mechanical assemblies.

Cotter pins provide a simple and effective means of fastening that can easily be removed or replaced when necessary, ensuring adaptability in mechanical applications. While cotter pins may play a role in preventing movement, their primary function is to keep components secured and aligned properly within a mechanical system.

The other options imply functions that are typically not the primary purpose of a cotter pin. For instance, fastening two components together more reliably involves other types of fasteners, while preventing rotation usually requires specific design elements like keyways or locking mechanisms. Aligning shafts in a coupling is achieved through different types of components designed for that purpose, rather than relying solely on a cotter pin.