What type of tool is commonly used for tightening or loosening screws?

A wrench is the correct tool for tightening or loosening screws, especially those with a hexagonal head, such as bolts or nuts. Wrenches provide the necessary grip and leverage to turn screws securely without stripping their heads. They're designed to fit over the head of the screw or bolt, allowing the user to apply torque effectively.

The other tools do not serve this purpose. A saw is meant for cutting materials like wood or metal and cannot apply the necessary force to turn screws. A hammer is utilized for driving nails or breaking objects apart, but it does not facilitate the twisting motion required to tighten or loosen screws. Pliers can grasp and turn certain types of fasteners, but they do not provide the precision and torque that a wrench does, making them less effective for this particular task.