Which of These Organisms Would Be Considered a True Autotroph

Which of these organisms would be considered a true autotroph?

A) A dodder that pierces into the phloem of its host plant to obtain nutrients
B) A mistletoe that receives energy from both photosynthesis and its host plant
C) An oak tree that receives all of its energy through photosynthesis
D) A sea slug that obtains nutrients from the algae it feeds on