Fail-closed solenoid operated valves open when:

A fail-closed solenoid operated valve is designed to remain closed unless a specific condition is met, allowing for the control of fluid flow in systems where safety is paramount. When the coil of the valve is electrically energized, it creates a magnetic field that pulls the valve's plunger or actuator, thus opening the valve.

This method of valve operation is important in scenarios where the default position of the valve is closed to prevent any undesirable flows when power is lost or in case of a system failure, which enhances safety and reliability. Therefore, the energization of the coil is what enables the valve to open, aligning perfectly with the fail-closed characteristic.

Understanding the operation of other types of valves can provide context for why the correct mechanism for a fail-closed solenoid valve is the energization of the coil, as options relating to motor operation, pressure changes, or de-energization do not accurately reflect the operational principles of solenoid valves.