How many half-lives are required for three-fourths of the nuclei of an isotope in a sample of decay?

Half-life is defined as the time it takes for half of a population to decay. if 3/4 of a sample has decayed, then it leaves 1/4 of the sample still intact. Then it is simple math. 1 half-life means only 1/2 of the population is still intact. Another half-life means only half of the (1/2) population is still intact, ie 1/4. So the answer is c)