Final answer:
The answer is 'Main vent', which connects several vent stacks and stack vents, extending through the roof for the escape of gases.
Step-by-step explanation:
The correct answer is c. Main vent. A main vent can connect several vent stacks and stack vents, allowing gases to escape through the roof to the atmosphere. It serves as an important part of a plumbing system to prevent the buildup of pressure and ensure proper drainage.