What was added to some state laws to ensure that whites who were illiterate or poor could still vote?

The correct answer is the grandfather clause.

After Reconstruction, many southern states put in place literacy tests and poll taxes to stop African-Americans from voting. However, these obstacles negatively effected poor white citizens as well. To combat this, many states implemented the grandfather clause. This clause states that if your grandfather was able to vote in the last election, than you can vote. This allowed poor whites to vote while still excluding African-Americans.