Final answer:
An arm extension is the correct term for the random length of pipe that a trap arm will penetrate through a wall at the proper height below a fixture.
Step-by-step explanation:
A trap arm, which is the part of the plumbing that runs from the p-trap to the vent, will penetrate through a wall at the proper height below the fixture with a random length of pipe called an arm extension. The correct answer to the question is d. Arm extension.
The trap arm and arm extension are essential in plumbing to ensure that waste water flows properly and prevents gases from the sewage system from entering the living space. Understanding the different components like the trap arm and arm extension is crucial for those studying basic engineering concepts related to plumbing.