Which of the following is not acceptable for enclosing underground service conductors?

The correct response highlights that ENT (Electrical Nonmetallic Tubing) is not acceptable for enclosing underground service conductors. This is largely due to its properties and the environments where it is typically used. Unlike other options, which are specifically designed to protect conductors in various conditions, ENT does not possess the necessary durability and moisture protection for underground applications.

When it comes to conduits used for underground service conductors, they must be resistant to mechanical damage, moisture, and other environmental factors. PVC (Polyvinyl Chloride) and RMC (Rigid Metal Conduit) are both well-regarded for their strength and protective qualities, making them appropriate for underground installations. Flexible metal conduit can also be used under certain conditions, though it generally offers less mechanical protection compared to RMC.

Understanding the limitations of each type of conduit is crucial for ensuring safety and compliance with electrical codes, which require specific materials for different applications.