The __ rule of law holds that a person can only defend a third party under circumstances and only to the degree that the third party could act. retreat stand your ground alter ego M'Naghten

## Step 1<br />The problem is asking us to identify the correct legal principle that allows a person to defend a third party under certain circumstances and to the extent that the third party could act. The options provided are "retreat", "stand your ground", "alter ego", and "M'Naghten".<br /><br />## Step 2<br />The principle of "alter ego" is a legal doctrine that allows a person to act on behalf of another person under certain circumstances. This doctrine is based on the idea that the person is acting as a substitute for the other person, or in other words, they are acting in the other person's "alter ego".<br /><br />## Step 3<br />The other options, "retreat", "stand your ground", and "M'Naghten", do not fit the description given in the problem. "Retreat" refers to the obligation to withdraw from a confrontation when it is safe to do so. "Stand your ground" means that a person has no duty to retreat and may use force in self-defense. "M'Naghten" is a test for insanity in criminal law.