The half-life of a substance describes the time in which it takes for half of the radioactive isotope to decay into another form. In your question, it says that the half life of radon-222 is 3.82 days; in a 100g sample of radon, 3.82 days pass and 50g of that isotope is left.
7.64 days is the equivalent to two half lives (7.64/3.82), so only half of half of 100%, or 25%, of the isotope would remain. The answer is D
Hope that helps
