What is not a clue that the oil tank is buried outside?
a)oil filter & shut off valve emerge from basement wall
b)filler pipe and vent are not right next to wall outside
c)you cannot find the oil tank inside
d)the flue is high efficiency plastic

I'm thinking B but I'm not sure

What is not a clue that the oil tank is buried outside?
a)oil filter & shut off valve emerge from basement wall
b)filler pipe and vent are not right next to wall outside
c)you cannot find the oil tank inside
d)the flue is high efficiency plastic
I'm going with "D"
I've heated with oil while living in New England and all the other options were present...
Where does this lead you?

Sounds like to a passing grade.

D. Oil cannot use the plastic flue. Oil will burn at to high a temperature and melt the plastic. Worst case is that the flue catches fire from old deposits. I have seen the galvanized run off flue pipe before when there was a uncontrolled fire in the flue and chimney.