Custody is defined by the Supreme Court as happening when:

A. A person has been taken into "custody."
B. A person has been formally arrested.
C. A person has been derived of his freedom of action in any significant way.
D. All of the above.

The correct answer is D. All of the above.

According to the Supreme Court's interpretation, custody occurs when any of the following conditions are met:
- A person has been taken into "custody," which generally refers to physical restraint or confinement.
- A person has been formally arrested by law enforcement.
- A person has been deprived of their freedom of action in any significant way, meaning their ability to move and act freely is restricted to a significant extent.

All three conditions mentioned in options A, B, and C indicate situations in which a person can be considered in custody according to the Supreme Court's definition.

To determine the correct answer, let's break down the options and analyze them one by one:

A. A person has been taken into "custody."
This option suggests that custody occurs when a person is taken into custody. It implies that physical restraint is necessary for custody to be established.

B. A person has been formally arrested.
This option states that custody only occurs when a person has been formally arrested. It implies that legal action must be taken by law enforcement for custody to be established.

C. A person has been deprived of his freedom of action in any significant way.
This option suggests that custody can occur when a person's freedom of action is significantly restricted. It implies that any substantial limitation on an individual's freedom could constitute custody.

D. All of the above.
This option claims that all the statements from options A, B, and C are true and that custody can happen in any of these scenarios.

Now, to determine the correct answer, we need to rely on the definition provided by the Supreme Court. Since the question refers to the Supreme Court's definition, we should consult the Court's interpretation.

To find the Supreme Court's definition of custody, you could start by conducting an online search using keywords such as "Supreme Court definition of custody." Look for reliable sources such as official Supreme Court rulings or legal websites to find a precise definition.

By exploring the relevant sources, you should be able to identify which option correctly aligns with the Supreme Court's interpretation of custody. Remember to validate the information based on reliable and credible sources.