Which of the following is NOT a lon-term trend marking the expansion of suffrage in the U.S.?

a. removing restrictive requirements based on religious belief
b. the Federal Government taking less of a role in protecting suffrage rights
c. eliminating requirements based on race
d. eliminating requirements based on tax payments

Since this is not my area of expertise, I searched Google under the key words "US suffrage rights" to get these possible sources:

http://caselaw.lp.findlaw.com/data/constitution/amendment19/
http://en.wikipedia.org/wiki/Suffrage
http://www.britannica.com/eb/article-233795/United-States

I hope this helps. Thanks for asking.