The correct answers are:

Ensure the root user uses a strong password.
Enable multi-factor authentication to the root user.

Explanation:
The root user has full access to all resources in an AWS account, so it's essential to secure it. AWS recommends the following best practices for root user access:

Use a strong password to reduce the risk of brute-force or credential-stuffing attacks.
Enable multi-factor authentication (MFA) to provide an additional layer of security in case credentials are compromised.

Why the other options are incorrect:

Storing root user access keys in an encrypted S3 bucket still poses a security risk and is discouraged. Best practice is to avoid creating access keys for the root user altogether.
Adding the root user to a group is not possible. IAM groups are for IAM users only; the root user is not managed like an IAM identity.
Applying inline policies to the root user is also not applicable. The root user inherently has full access and cannot have IAM policies attached to it.

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