The least accurate definition of the term "sooner" is:
B. a land run claimant who registered for participation too early
This definition is misleading because "Sooners" specifically refers to individuals who claimed land before the official opening of the land run, rather than those who simply registered early. Choices C and D directly relate to the definition of "Sooners," and A is somewhat tangential but not entirely incorrect. Therefore, B is the least accurate.