Answer
Choice E.
Work Step by Step
After each half-life, one-half of the remaining material decays.
After one half-life, one day, we are down to one-half the original amount.
After two half-lives, 2 days, we are down to one-quarter the original amount.
After three half-lives, 3 days, we are down to one-eighth the original amount.
This means that 7/8 of the original sample has decayed.