You have an application running on an EC2 instance which will allow users to download files from
a private S3 bucket using a pre-signed URL. Before generating the URL, the application should
verify the existence of the file in S3.
How should the application use AWS credentials to access the S3 bucket securely?

A.
Use the AWS account access keys; the application retrieves the credentials from the source code
of the application.

B.
Create an IAM role for EC2 that allows list access to objects In the S3 bucket; launch the Instance
with the role, and retrieve the role’s credentials from the EC2 instance metadata.

C.
Create an IAM user for the application with permissions that allow list access to the S3 bucket;
the application retrieves the IAM user credentials from a temporary directory with permissions that
allow read access only to the Application user.

D.
Create an IAM user for the application with permissions that allow list access to the S3 bucket;
launch the instance as the IAM user, and retrieve the IAM user’s credentials from the EC2
instance user data.

Explanation:
http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-instance-metadata.html