Final answer:
Shoulder flexion and elbow extension provide the best stretch to the long head of the triceps.
Step-by-step explanation:
The end range position that would provide the BEST stretch to the long head of the triceps is shoulder flexion and elbow extension (B).
When the shoulder is flexed, the long head of the triceps is placed on stretch, and when the elbow is extended, it further elongates the muscle, creating a greater stretch.
For example, when you reach overhead and straighten your arm, you can feel a stretch in the triceps.