Could be true. We have no information about how short-term memory skills exhibited by nonsmokers who have not recently smoked a cigarette compare with those exhibited by nonsmokers who have recently smoked, so we can’t conclude that (A) must be false.

Must be false. This directly refutes the information in the stimulus: we know that the short-term memory skills exhibited by the typical nonsmoker who has just smoked a cigarette will be worse than those exhibited by the typical regular smoker who has just smoked a cigarette!

Could be true. We don’t know how nonsmokers’ short-term memories stack up against smokers’ short-term memories once those smokers have gone over 8 hours without a cigarette!

Could be true. The stimulus does nothing to rule out the possibility that the regular smoker described in (D) could experience a boost in short-term memory skills after smoking heavily, even if their short-term memory skills were not superior immediately after one cigarette.

Could be true. We have no information about what happens to smokers’ superior short-term memories during the five hours after their last cigarette—maybe they improve before dropping off later!