Answer: (A)
An explanation for Zahra's lapse in short-term memory is that "she is reacting to a new medication her doctor recently prescribed."
Explanation:
Several prescribed drugs have side effects which include memory loss. Some of such drugs include; sleeping pills, antidepressants, blood pressure medication, arthritis medication, among others.
Older adults take longer to breakdown and absorb medication and as such are more prone to short term memory losses due to these medications.
Therefore Zahra who is seventy-eight years old is most likely "reacting to a new medication her doctor recently prescribed."