Final answer:
States weakened the Fifteenth Amendment by imposing poll taxes, which were barriers that economically disenfranchised many African Americans and other racial minorities. Therefore the correct answer is b. by requiring individuals to pay poll taxes for the right to vote
Step-by-step explanation:
One way states were able to weaken the effects of the Fifteenth Amendment was by requiring individuals to pay poll taxes for the right to vote. The Fifteenth Amendment stated that the right to vote could not be denied based on race, color, or previous condition of servitude, but it did not address barriers like poll taxes or literacy tests. As a result, despite the formal grant of voting rights, these economic and educational barriers served to disenfranchise many African Americans and other racial minorities.