Final answer:
Maximum medial/lateral COM movement occurs at heel strike, as the body initiates contact with the ground and starts to stabilize before bearing weight on the leg.
Step-by-step explanation:
The maximum medial/lateral center of mass (COM) movement occurs at heel strike (d). During heel strike, when the foot first makes contact with the ground, the body begins to adjust its balance, causing a shift in the COM to stabilize. This initial contact generates the greatest deviation in the medial/lateral direction as the body prepares to bear weight on that leg.
In contrast, at mid-stance (a) the body is typically at its most stable position with the COM over the base of support. Toe-off (b) marks the transition from stance to the swing phase, and while there is movement, it's not the maximum medial/lateral COM movement. Finally, during mid-swing (c), the COM is moving forward with the limb in motion; thus, there is minimal lateral shift compared to heel strike.