Final answer:
The method used to determine that no cross-connection exists after the completion of installer tests is either individual pressurization or pressure differential.
Step-by-step explanation:
After the completion of all installer perform tests, the method that should be used to determine that no cross-connection exists is D) either A or B. Individual pressurization and pressure differential are two viable methods for assessing the integrity of installations such as pipeline systems, valves, and other components that need to maintain a specific separation of fluids or gases. The choice between these methods can depend on the specific requirements of the system being tested, the preferences of the engineering team, or industry standards.
Individual pressurization involves applying pressure to one section or component at a time to ensure that there is no leakage or cross-connection to any other section. On the other hand, pressure differential testing is the process of applying different pressures to two sides of a system to detect any unintended cross-connections through changes in pressure readings.