Which option is the cause of this issue?

Explanations:

The new instance type specified in the CloudFormation template is likely valid, as the stack entered an UPDATE_ROLLBACK_FAILED state, indicating that the template was processed before the rollback was initiated. If the instance type were invalid, it would have failed the update immediately.

If the database was deleted or modified manually outside of the CloudFormation stack, it can lead to the stack entering the UPDATE_ROLLBACK_FAILED state because CloudFormation cannot reconcile the state of the stack with the actual resources in AWS. This discrepancy prevents the update from completing successfully.

A syntax error in the CloudFormation template would generally prevent the stack update from being initiated at all, resulting in a different error state, such as CREATE_FAILED or UPDATE_FAILED, rather than UPDATE_ROLLBACK_FAILED.

Insufficient IAM permissions would typically prevent the stack update from being executed initially. If permissions were the issue, it would result in a failure state rather than an UPDATE_ROLLBACK_FAILED state. The stack would not attempt to update the instance at all.