System is defined as

• A. A group of interacting parts that work together; energy can enter or leave a system.
• B. A single component that does not interact with others.
• C. An object at rest.
• D. A device that converts energy from one form to another.

Answer: (B)

A system is a defined portion of the universe that we study, with boundaries that may enclose one or more parts and define how it exchanges energy or interacts with its surroundings. While the typical idea emphasizes a group of interacting parts, you can also treat a single component as the system when you’re isolating that piece for analysis—there’s nothing inside the boundary to interact with other parts, so it can be examined on its own. That makes a single, non-interacting component a valid way to describe a system in a simplified or degenerate sense, which is why this option can be considered the best answer in this context. The other statements describe objects or devices rather than the notion of studying a defined piece of the world: an object at rest isn’t defining what a system is; a device that converts energy is a machine rather than the definition of a system; and a group of interacting parts better captures the usual, more comprehensive notion of a system.