Solution: Elasticity

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Coefficient of Restitution:
e = -(v_1f - v_2f)/(v_1i - v_2i)

Because basketball is the only object moving, v_2i and v_2f are both 0 and hence neglected in the equation.
e = -v_f/v_i
v_f² = 2gDx

v_f = -10m/s (in the opposite direction when bounced-off)
v_i = 9m/s
e = 9/10 = 0.9

Answer: 0.9