According to Security Best Practices, How Should an Amazon EC2

According to security best practices, how should an Amazon EC2 instance be given access to an Amazon S3 bucket?

A) Hard code an IAM user's secret key and access key directly in the application, and upload the file.
B) Store the IAM user's secret key and access key in a text file on the EC2 instance, read the keys, then upload the file.
C) Have the EC2 instance assume a role to obtain the privileges to upload the file.
D) Modify the S3 bucket policy so that any service can upload to it at any time.