Answer:
The correct answer is "Precedent"
Step-by-step explanation:
The whole concept of common law is based in the principle of precedent, which is a legal principle that establish that judges are obliged to follow a certain decision made by a superior court or an equal competent one, following the Latin maxim "Stare decisis et non quieta movere" that translates "to stand by decisions and not disturb the undisturbed" this latin maxim is strongly binded to the roots of common law, so in order to decide the outcome of a similar case in the future they use a previous decision or precedent in order to preserve the sense of the law previosly determined.
As an addition, even though the precedent is binding it's not obbligatory, a judge can ignore the precedent if he has a strong reason, remember law is in perpetual motion and it changes through times, but this principle can not be ignored.