The correct answer is: C. Isolated joint with all the forces, internal as well as external, acting on it.
A free body diagram (FBD) is a diagram of a body or system of bodies that is isolated from its surroundings and is subjected to only external forces. The forces acting on the body are represented by arrows, and the direction of each arrow indicates the direction of the force. The magnitude of each force is represented by the length of the arrow.
Internal forces are forces that act between two parts of the same system. They are not shown on FBDs because they cancel each other out. External forces are forces that act on the system from outside. They are shown on FBDs.
The following is a brief explanation of each option:
- Option A: An isolated joint with only body forces acting on it is not a free body diagram because it does not include all of the forces acting on the system.
- Option B: An isolated joint with internal forces acting on it is not a free body diagram because it does not include all of the forces acting on the system.
- Option C: An isolated joint with all the forces, internal as well as external, acting on it is a free body diagram because it includes all of the forces acting on the system.
- Option D: None of the above is not a correct answer because it does not include any of the options that are correct.