What type of door trigger circuit shows +12V when closed and 0V when the door is open?

The correct choice represents a door trigger circuit configuration that operates with a normally closed design, meaning that under normal conditions (when the door is closed), the circuit allows current to flow and thus maintains a voltage of +12V. This setup is beneficial because it is designed to send a signal when the door is in its closed position.

When the door is opened, the circuit is interrupted, breaking the connection and causing the voltage to drop to 0V. This transition from high to low voltage can trigger an alarm or notification system, making it particularly effective for security applications.

This behavior is characteristic of a negative trigger circuit, and in this configuration, the switch or relay operates differently than a normally open or positively triggered switch. In a normally open design, there would be no voltage when the door is closed. The involvement of wiring and grounding also influences how the voltage is read in these scenarios, which is critical for understanding the operation of such systems.