Which task can only an AWS account root user perform?

Explanations:

Only the root user can change the AWS Support plan. Other IAM users can manage resources but lack the permissions to modify the support plan settings.

Deleting AWS resources can be performed by users with the appropriate IAM permissions. The root user is not required for this task unless specific restrictions apply.

Creating an Amazon EC2 instance key pair can be done by any user with the necessary permissions in IAM. The root user is not exclusively required for this.

Configuring AWS WAF (Web Application Firewall) can be done by IAM users who have been granted the necessary permissions. It is not restricted to the root user.