Which of the following tools is commonly used for tightening or loosening bolts in a small engine?

A socket set is specifically designed for tightening and loosening bolts and nuts, making it the most effective choice among the provided options for small engine work. Socket sets include various socket sizes that fit different bolt heads, allowing for better leverage and a more secure grip than other tools. This ensures that the fasteners can be tightened to the appropriate torque specifications, which is especially important for maintaining engine integrity.

An adjustable wrench, while it can be used for bolts, does not provide the same level of fit or torque as a socket. It can slip off the fastener if not positioned correctly, potentially damaging the bolt or surrounding components. Pliers, on the other hand, are designed primarily for gripping and cutting rather than applying the necessary torque required for bolts. Hammers are not used for this purpose at all and are typically employed for driving or breaking apart components rather than for fastening.

Overall, a socket set is the preferred tool for the task at hand due to its design that accommodates a variety of bolt sizes and provides the necessary torque without damaging the fasteners.