What must an illegal substance be for someone to be considered in possession of it?

For someone to be considered in possession of an illegal substance, it must be in their possession or under their control. This definition involves not only physical possession, meaning the individual has direct access to the substance, but also constructive possession, where the substance is not in their immediate control but they have the ability to exercise control over it. This principle is crucial in legal contexts, as it establishes accountability and jurisdiction over the illegal item.

The other choices do not accurately reflect the requirements for possession of an illegal substance. Visibility to the public does not determine possession; in fact, individuals may possess substances that are not visible at all. Similarly, locking a substance in a container does not negate possession; it may simply indicate that the substance is stored or secured. Finally, while laws specify which substances are illegal, the mere existence of a law does not satisfy the condition of possession, as possession is fundamentally about control and access. Thus, the criterion that an illegal substance must be in a person's possession or under their control is essential for establishing possession.