The infamous "grandfather clauses" of the late 19th and early 20th centuries stated that

a)
only the grandfathers of ex-slaves would be allowed to register to vote.

b)
only grandfathers would be allowed to register to vote in coming elections.

c)
all the grand children of registered voters after the civil war would be allowed to vote.

d)
only people whose grandfathers could vote before the civil war would be allowed to register to vote.

The correct answer is D.

In the Reconstruction Era, the 14th and 15th amendments to the US Constitution had been introduced in order to guarantee equal rights for all US citizens, preventing any form of discrimination in terms of race.

The states could not explicitly prevent any citizen from participating in the elections. Still, many Southern states, circumvented the newly established constitutional provisions by implementing new requirements such as literacy tests, payment of poll taxes, property restrictions, etc., that needed to be fulfilled in order to register to vote. These measures excluded mostly black citizens as many were poor and/or illiterate.

Moreover, the grandfather clauses were introduced. These were used so that those whose ancestors were able to vote before the Civil War, could continue doing it wihtout the need of proving that they met the extra requirements: the tests, the poll taxes, etc. These provisions enahnced even more the discrimination against black US citizens.

D) only people whose grandfathers could vote before the Civil War would be allowed to register to vote.

Explanation:

Grandfather clauses, allowed a man to vote if his grandfather or father had voted before January 1, 1867, this exemption clause effectively denied all freedmen the ability to vote. Free-colored men before 1831 could vote in North Carolina if they met the property requirements, however they were excluded there and elsewhere after the slave rebellion of 1831.