Final answer:
The phenytoin 100 mg with an expiration date of September 2020 expires on September 30, 2020, which is the last day of the month as per standard pharmaceutical practice.
Step-by-step explanation:
The expiration date on a medication indicates the last day that the manufacturer guarantees the full potency and safety of a medication. In the case of the phenytoin 100 mg with an expiration date of September 2020, the medication would expire on September 30, 2020. This is standard practice for pharmaceuticals; unless a specific day is indicated, the expiration date is the last day of the month. Therefore, the correct answer to the question about the expiration date of the phenytoin is c) September 30, 2020.