Answer:
The correct answer is a. It isn't possible to have the same login ID for multiple IAM users of the same account.
Explanation:
When you first create an AWS account, you start with a single login identity that has full access to all AWS services and resources on the account. This identity is named AWS for the root user account and is accessed by logging in with the email address and password that you used to create the account. We recommend that you do not use root user in your daily tasks, not even in administrative tasks. Instead, it is best to stick to the best practice of exclusively using root user to create the first IAM user. Then keep the root user credentials in a safe place and use them only for some account and service administration tasks.