What was the grandfather clause?

a it gave a person the right to vote if their father or grandfather had been a voter before the civil war.

b it gave a person the right to vote if they had a father or grandfather alive before the civil war.

c it allowed a person to vote only if their father was in the voting booth along side of them.

d it allowed a person to vote if their grandfather had voted before the constitution of 1898 was adopted.