Final answer:
The approved conduit for direct burial is PVC conduit, due to its resistance to corrosion and moisture. EMT and Flexible Metal Conduits are not suitable for this purpose, while Rigid Metal Conduit may require extra protection.
Step-by-step explanation:
The type of conduit approved for direct burial is the PVC conduit. PVC, or Polyvinyl Chloride, is a type of plastic conduit that is suitable for underground applications due to its resistance to corrosion, chemical rot, and damp soil conditions. It is an ideal choice for outside underground electrical lines. While there are different types of conduits available, not all are suitable for direct burial. For example, EMT (Electrical Metallic Tubing) and Flexible Metal Conduit are not designed for direct burial, as they can corrode when in contact with the soil and elements. Some Rigid Metal Conduits can be buried directly, but these would typically require additional protection or coatings to prevent corrosion.