What is a possible reason for this?

You are signed in as root user on your account but there is an Amazon S3 bucket under your account that you cannot access.

What is a possible reason for this?
A . An IAM user assigned a bucket policy to an Amazon S3 bucket and didn’t specify the root user as a principal
B . The S3 bucket is full.
C . The S3 bucket has reached the maximum number of objects allowed.
D . You are in the wrong availability zone

Answer: A

Explanation:

With IAM, you can centrally manage users, security credentials such as access keys, and permissions that control which AWS resources users can access.

In some cases, you might have an IAM user with full access to IAM and Amazon S3. If the IAM user assigns a bucket policy to an Amazon S3 bucket and doesn’t specify the root user as a principal, the root user is denied access to that bucket.

However, as the root user, you can still access the bucket by modifying the bucket policy to allow root user access.

Reference: http://docs.aws.amazon.com/IAM/latest/UserGuide/iam-troubleshooting.html#testing2