The strongest reasoning the author gives to support his claim is:
C. Unfair laws can encourage White people to stay uneducated and living in poverty.
In the excerpt, Washington argues that the unjust law which allows ignorant and poverty-stricken white men to vote while disenfranchising equally situated Black men will have negative effects not only on the Black community, encouraging them to seek education and property, but also on white individuals, potentially fostering a cycle of ignorance and poverty among them. This highlights the broader implications of such laws.