Step 1: Concept
Steering geometry involves several angles that improve handling, stability, and tire wear.
Step 2: Meaning
This specific angle describes the "tilt" of the steering axis when viewed from the side.
Step 3: Analysis
The caster angle is the tilt of the king-pin (the pivot point) from the vertical. Positive caster provides self-centering for the steering, making the tractor easier to drive in a straight line.
Step 4: Conclusion
The described angle is the caster angle.
Final Answer: (D)