Snap rings hold and fit into grooves in:

Snap rings are mechanical fasteners designed to secure components, such as bearings or gears, onto shafts or within housings. They fit into grooves, providing a form of retention that prevents parts from moving axially along a shaft or pin. This design helps maintain the proper alignment and spacing of parts within various assemblies.

When considering the options provided, snap rings can indeed hold and fit into grooves on pins, gears, and shafts. They serve essential functions across all these components. Therefore, the answer encompasses a broad applicability of where snap rings are utilized within mechanical systems. This versatility in deployment makes them critical in ensuring that various parts function effectively and remain securely in place under operational stresses.