答案：D

解析：

Choice D is the best answer. Obama argues that "we've still got problems" related to voting, and explains what those problems are: the Voting Rights Act has been "weakened," which has "the effect of discouraging people from voting," and this means that "our democracy erodes." These claims all lead up to the passage's central claim: Congress should "pass an updated version of the Voting Rights Act that would correct" these problems.