The quote from President Jackson, "John Marshall has made his decision... Now let him enforce it!" indicates that Jackson believes that while Justice Marshall's ruling has been made, it lacks practical enforcement without the support of the executive branch, which he controls. This implies a disregard for the ruling's authority and suggests that Jackson does not support Marshall's decision.
Given this context, the best interpretation of Jackson's statement is:
D John Marshall cannot force settlers not to settle on Native American lands.
This choice reflects Jackson's assertion that Marshall's decision lacks the means of enforcement and highlights his belief that the executive branch, in this case, the administration led by Jackson, would not abide by the ruling that favored Native American rights.