Final answer:
Even if a system has zero acceleration, it can still have balanced external forces and internal forces. Only if the external forces are balanced can acceleration be zero, while internal forces can exist in a non-accelerating system.
Step-by-step explanation:
If the acceleration of a system is zero, it does not necessarily mean that no external forces are acting on it. A system can have external forces acting on it that are balanced, meaning they cancel each other out, resulting in no net external force and thus no acceleration, according to Newton's first law of motion. However, there might still be internal forces within the system, which also cancel each other out. For example, think of a book resting on a table; the gravitational force acting on the book (an external force) is balanced by the normal force from the table, so the book does not accelerate. But there are internal forces at play maintaining the structural integrity of the book and table, even though they do not contribute to the system's acceleration.
Regarding internal forces, even if a system is not accelerating, it can still have internal forces. These forces could be structural, as in the forces holding together the components of a stationary object, or dynamic, such as the tension in parts of a machine that is operating at a steady state.
Therefore, the correct answer to the question is: d) External forces are zero; internal forces may or may not be zero.