How Does IAM Explicit Deny Affect EC2 Instance S3 Access?

This question tests two distinct but related AWS concepts: the AWS credential provider chain and IAM policy evaluation logic.

First, let's address the credential provider chain. When the AWS CLI or SDK runs on an EC2 instance, it looks for credentials in a specific order: environment variables, then the credentials file (~/.aws/credentials), and finally the EC2 instance profile (IAM role). In this scenario, because the credentials file exists and contains valid keys, the AWS CLI will indeed use those keys, not the IAM role's temporary credentials. This is where many candidates get tripped up, as noted by user Kb80 in the comments.

However, the critical part of this question is the second concept: IAM policy evaluation logic. Once the AWS CLI has selected a set of credentials (in this case, the ones from the credentials file), it makes an API call to S3. AWS then evaluates all policies attached to the identity associated with those credentials. The fundamental rule of IAM policy evaluation is that an explicit deny always wins. If any policy attached to the user whose keys are in the credentials file contains an explicit deny for S3 actions, that deny will block the request, regardless of any other allow statements in any other policies.

The question states that the IAM role attached to the EC2 instance explicitly denies S3 access. While the CLI might use the keys from the credentials file, the evaluation of those keys' permissions is what matters. If the user associated with those keys also has an explicit deny (or if the question implies the role's deny somehow applies to the session), the action is blocked. The key takeaway, as emphasized by tomchandler077 and Anandesh, is that an explicit deny is the ultimate trump card in IAM. It doesn't matter how many "Allow" policies you have; a single explicit deny will shut down access. Therefore, the EC2 instance will not be able to perform any S3 action, making D the correct answer.