Final answer:
Max ER occurs during the heel strike phase of gait.
Step-by-step explanation:
The correct answer is d. Heel strike. Max ER, which stands for Maximum External Rotation, occurs during the heel strike phase of gait. This phase occurs when the heel of the foot makes initial contact with the ground while walking or running. At this moment, the foot is externally rotated to allow for a smooth transition from the swing phase to the stance phase of gait.