An extensor support moment is the part of the stance phase

a. during toe-off when the back knee bends and the weight of the lower limb initiates the swing phase.
b. during the heel strike when the spine extends as a protective measure to dissipate the shock of heel compression.
c. when the body loses its vertical balance and the knees and hips of both lower limbs lock simultaneously to prevent falling.
d. when hip and knee extend to prevent the lower limb from buckling as the body leans forward .

d