Final answer:
The half-life of a radioactive substance is the time it takes for half of it to decay. It's a key concept for predicting the activity and longevity of radioactive isotopes.
Step-by-step explanation:
The half-life of a radioactive substance is the amount of time it takes for half of the original amount of the substance to undergo radioactive decay. Option b) is correct: it is the time it takes for the substance to decay to half of its initial amount. This concept is crucial in understanding the decay behavior of radioactive isotopes, and it helps scientists predict how long a radioactive substance will remain active. Different isotopes have different half-lives ranging from microseconds to billions of years. After one half-life, there's 50% of the original substance remaining; after two half-lives, 25% remains, and this pattern continues with each subsequent half-life.