Wing anti-ice valves close automatically when:

The correct answer highlights an important safety feature of the wing anti-ice system. When a leak is detected in the anti-ice system, it triggers an automatic closure of the wing anti-ice valves. This action is crucial because a leak can compromise the effectiveness of the anti-ice system, leading to ice formation on the wings and impacting flight safety.

Detecting a leak and closing the valves ensures that the system remains effective and that the aircraft operates safely, preventing potential performance issues due to ice accumulation.

The other choices involve conditions that do not specifically mandate the closure of the valves. For example, valve position discrepancies and high-pressure conditions may indicate issues but do not inherently trigger an automatic closure for safety. High temperature might suggest that the anti-ice system is functioning properly, as it typically operates best within certain temperature parameters. Thus, the automatic closure mechanism is specifically designed as a response to leak detection, making it the most relevant condition for valve closure in anti-ice systems.