Which feature enhances the safety of a firearm when it is not in use?

A safety lock is an essential feature that enhances the safety of a firearm when it is not in use because it prevents the firearm from being fired accidentally. By engaging the safety lock, the shooter can ensure that the trigger cannot be pulled, thereby avoiding unintentional discharge of the firearm. This is particularly crucial when the firearm is stored or handled by someone who may not be authorized or trained to use it.

In contrast, high-capacity magazines allow for more rounds to be stored in the firearm, which does not contribute to safety when the firearm is not in use. An adjustable stock refers to the customization of the firearm for a better fit and comfort for the shooter, but again does not provide any safety features when the firearm is not actively being used. A Picatinny rail is a mounting system for accessories such as scopes or lights, which can enhance functionality but does not address preventing accidental discharge. Thus, the safety lock stands out as the primary feature for ensuring that a firearm remains secure and safe when it is not in operation.