Which of the following best defines a thermodynamic system?

The definition of a thermodynamic system is best captured by identifying it as a collection of matter under study, separated from its surroundings. This means the system is a defined quantity of matter or a physical region in space that is specifically of interest for analysis in thermodynamics.

This separation from the surroundings is essential because it allows for the examination of energy and matter interactions without complications introduced by external factors. The behavior of the system can be influenced by the energy and matter transfers that occur across its boundaries, and understanding these interactions is crucial for applying the laws of thermodynamics.

In contrast, the other options misrepresent what constitutes a thermodynamic system. They either inaccurately describe the system's boundaries or its interactions with the surroundings, which are key aspects in thermodynamics. Therefore, option B accurately and adequately defines a thermodynamic system by emphasizing its nature and the focus of study.