In the expression, 4,000(0.58)^3n...
4000 would represent the initial amount of fluid in the patient's lungs before the drug is introduced (it is the maximum value as well).
0.58 would represent the rate at which the amount of fluid in the patient's lungs is removed by the drug (42% removed every 8 hrs).
3n would represent the number of times the drug has removed a certain percentage of fluid from the patient's lungs ("n" by itself would actually represent the amount of days elapsed).
The whole expression (4,000(0.58)^3n) would represent the amount of fluid remaining in the patient's lungs after each day.
answer: C