Which solution will meet these requirements?

Explanations:

Configuring permissions on individual files does not provide a secure way to ensure that only CloudFront can access the files. It requires managing permissions for each file separately, which can be cumbersome and prone to errors. Moreover, files could still be accessed directly if not correctly set.

Creating an origin access identity (OAI) and associating it with the CloudFront distribution allows CloudFront to access the S3 bucket while keeping the bucket content private. The S3 bucket can then be configured to grant access only to the OAI, ensuring that direct access from the internet to the S3 bucket is denied.

Creating an IAM role for CloudFront to assume does not directly solve the problem of keeping S3 files private. While IAM roles are used for access management, they do not provide a direct way to restrict access to the S3 bucket itself without additional configurations. Furthermore, CloudFront does not assume IAM roles to access S3.

While creating an S3 bucket policy that uses the CloudFront distribution ID as the principal might seem like it restricts access, it is not a valid approach. The S3 bucket policies require either IAM users/roles or AWS services as principals, and using only the distribution ID does not enforce the necessary security without the OAI setup.