Final answer:
The statement is true, as Newton's third law states that for every action force there is an equal and opposite reaction force, applying this principle to external forces on a system.
Step-by-step explanation:
The statement that Newton's third law applies to the external forces acting on a system of interest is true. Newton's third law, often summarized as "for every action, there is an equal and opposite reaction," implies that in interactions between two objects, there are two forces at play that are of equal magnitude and opposite in direction. When considering a system of interest, it must be understood that the action and reaction forces do not act on the same object, but rather on different objects that are part of the system or interaction. For example, if you were to push a book, the book pushes back with an equal and opposite force. This illustrates that all external forces, as part of an interaction, follow Newton's third law, which is critical to understanding the fundamental principles of physics.