What should be done to ensure optimal security?

An application is running on an Amazon EC2 instance. A SysOps Administrator is tasked with allowing the application access to an Amazon S3 bucket.

What should be done to ensure optimal security?
A . Apply an S3 bucket policy to allow access from all EC2 instances
B . Create an IAM user and create a script to inject the credentials on boot
C . Create and assign an IAM role tor Amazon S3 access to the EC2 instance.
D . Embed an AWS credential file for an IAM user inside the Amazon Machine Image (AMI)

Answer: A