Cn you me with this question?

That period of time is called the "half-life" of that particular isotope,
because that's how long it takes for ONE-HALF of it to decay and
turn into some other substance.

So, however much you start out with, you can expect that after one
half-life of time has passed, half of the total amount (50%) has decayed
and turned into other substances, and the other half (50%) remains.

Ithink the answer is a