In united states v. ceccolini, the court created the so-called here they observed that witnesses "can, and often do, come forward and offer evidence entirely of their own volition."

Independent source exception

Explanation:

It is an exemption to the exclusionary rule.

In the law of the United States, this convention applies to evidences that are at first found during, or as a result of, an illegal search, but later on are obtained autonomously from the exercises that are unaffected by the underlying lawlessness.

It was observed that, in the doctrine of independent source, witness has the right and usually come forward with the proof on their own will.

independent source exception

Explanation:

The answer is --

independent source exception

          According to the United States law, an 'independent source' of evidence is a special case to the exclusionary rule of the law. This rule states that an evidence discovered during an unlawful and illegal search or investigation but which is later on available independently from the exercise which is undamaged by the initial or the previous illegality.

         In this particular lawsuit, the court created the "independent source exception" doctrine. And they observed that witnesses often come forward and offer evidence on their own will.

Hence the answer is --

"independent source exception"

it should be d or b but i'm sure it's d