Which of the following is the best definition of half-life for a radioactive substance?

Ok so lets say something takes 20,000 years to get rid of a half life that means in 20,000 years the radioactivity will be half of what it originally was and then in anothor 20,000 years the half of the half of what was originally there and so on. But the only problem is it will never be completely gone.

Here is a chart from Wikipedia and i know oh its not reliable well i checked there sources are correct check if you want.

Rate law for this reaction israte of the reaction = k[a]²[b].second order in a  means concentration of a in power of two, and  first order in b means concentration of b in power of one.