AWS Certified Solutions Architect Associate SAA-C03 Practice Question

The most secure and flexible way is to use an IAM role on the EC2 instances with permissions to access the S3 bucket, and set up cross-account access using a resource-based policy on the S3 bucket. This approach uses IAM roles to grant temporary credentials to the EC2 instances, avoiding the need to store long-term access keys. The resource-based policy on the S3 bucket allows the IAM role from the other account to access the bucket, enabling cross-account access while adhering to the principle of least privilege.

Creating an IAM user and storing access keys on EC2 instances is less secure because it involves managing long-term credentials, which can be compromised. This approach is not flexible either, as it involves access keys management in each EC2 instance.

Configuring the EC2 instance's security group affects network traffic but does not grant permissions to access S3 buckets; AWS permissions are managed via IAM policies, not security groups.

Copying the objects to a local S3 bucket duplicates data unnecessarily and requires additional management, hence not flexible. It also does not solve the cross-account access issue securely.