Which task requires using AWS account root user credentials?

Explanations:

Viewing billing information can be done by IAM users with appropriate permissions. The root user is not required for this task.

Changing the AWS Support plan requires root user credentials, as this action involves billing and account management settings that are restricted to the root user.

Starting and stopping Amazon EC2 instances can be managed by IAM users with the necessary permissions. Root user access is not needed for this operation.

Opening an AWS Support case can also be performed by IAM users who have been granted the necessary permissions. Root user credentials are not required for this task.