I disagree with both of your answers.
State legislators serve their individual states, thus limiting the power of the federal government.
I have to disagree with the esteemed MsSue on the first one: Originally, the "Senate of the United States shall be composed of two Senators from each State, chosen by the Legislature thereof for six Years; and each Senator shall have one Vote." The 17th Amendment changed the "chosen by the Legislature thereof" to popular vote in each state (many states had already directed that their state's Senators be selected by popular vote).
On the second on, IMO, a bad question. a limits, d limits, leaving b and c. Well voting rights limit the fed government, as many laws can be overturned, the constitution amended, and an adminsitration and legislator can be voted out.
On c, MsSue is right, however, I dont think the writer of the question considered that.
Thanks, Bob. I knew I should have looked that up rather than depend upon memory.
I see what threw me. The Senators were chosen by the state legislatures.