Carbon-14 has a half-life of approximately 5,730 years. how many half-lives have occurred in a sample that contains 25 percent of the original amount of the parent isotope?

two half lives

Explanation:

half life is the time taken for the radioactivity of an isotope to reduce to half of its original value

therefore, in the first half life carbon-14 would have reduced by 50% in 5730 years

second half life, the isotope would have reduced to 25% in 11460 years

in three half lives, 12.5% of the isotope will be left after 17190 years, and so on

conclusively, 25% would remain after two half-lives have occurred