Sorry for the confusion. My question comes from a practice test in the book, Stallcup's Journeyman Electrician's Study Guide.
The question verbatim is:
Rigid nonmetallic conduit shall be installed a least ________ in. below the top surface of finished grade.
I doesn't mention it's condition. So, I take it as minimum requirement for any and all conditions. Which would be 0 in. provided that it is under a building Table 300.5. But to pick an answer from those choices I would pick B. 12 in. with the condition that it is in a trench below 2 in thick concrete. What do you think? Really it's a bad question if they don't give you a condition.
The answers are published in another book, which I don't have. So, Occasionally I come across a question that can be ambiguous.