What will cause air to leak out of the relay valve with the park brake released and the service brake not applied?

The correct choice identifies the issue related to the air leaking from the relay valve when the park brake is released and the service brake is not applied. A leak at the parking/emergency brake chamber pushrod seal would allow air to escape, resulting in a pressure drop in the system.

When the parking/emergency brake is released, the pushrod is free, and any wear or damage to the seal can create a pathway for compressed air to escape. This scenario is critical because it indicates a failure in maintaining the necessary pressure for the brake system, potentially leading to brake performance issues.

In contrast, a ruptured service brake diaphragm would primarily affect the service brake operation, and a malfunctioning park/emergency brake diaphragm would turn into an issue specifically when the emergency brake is deployed. A faulty secondary reservoir blow-off valve would mainly influence air pressure retention in the reservoir rather than cause air to leak from the relay valve specifically under the circumstances described. Thus, a leak at the pushrod seal is the most direct reason for air escaping from the relay valve under the specified conditions.