Final answer:
Without additional context or terms of the credit agreement, it's impossible to determine the correct last date of the credit period from the options provided.
Step-by-step explanation:
To determine what the last date of the credit period is, we would need additional context surrounding the terms of the credit that was extended. Generally, in business, the credit period refers to the amount of time a buyer has to pay a seller for the goods or services purchased on credit. Without more information about when the credit period began or the terms of the credit agreement (such as a 30-day or 60-day period), it isn't possible to accurately determine the last date of the credit period solely from the options provided: a) September 30, 2021, b) October 15, 2021, or c) September 15, 2021.