The correct answer is E.

This question requires an understanding of the meaning of the term 'half-life' and a careful reading of the question stem. Since the half-life is given as six days, one-half of the sample will decay (and therefore the other half will remain) every six days; after 24 days, or four half-lives, (1/2)⁴, or 1/16, will remain. One sixteenth of 30 grams would give us choice A, but the question asked for how much will have decayed rather than how much would be left; the correct answer is therefore 15/16 of the original 30 grams, or choice E.