I believe the answer is: Experimental method
If professor garcia adopt an experimental method, Garcia needs to purposefully put the children into a situation where they had to get spanked. This would be seen as unethical because it could potentially cause an everlasting emotional scar for the children.
Professor Garcia would like to study parental spanking and aggressive behavior in children. The experimental method would not be feasible because of ethical problems associated with trying to manipulate the spanking variable.
