Final answer:
A trailer hand valve should not be used to hold a unit that will be left unattended because it may cause a pressure surge.
Step-by-step explanation:
The correct answer is b) It may cause a pressure surge.
A trailer hand valve is a device used to control the brakes on a trailer. It is not designed to hold a unit that will be left unattended. When the hand valve is engaged and left unattended, it can lead to a pressure surge in the trailer's brake system. This surge can cause damage to the brake components and compromise their effectiveness.
It is important to use the correct equipment and follow proper procedures when parking or leaving a trailer unattended to ensure the safety and integrity of the brake system.