A. autotrophs
B. heterotrophs
C. saprotrophs
D. none of these
The first organisms on Earth were most likely heterotrophs. These early life forms would have consumed organic molecules from their environment to obtain energy and nutrients for survival, as opposed to producing their own food through processes like photosynthesis (autotrophs) or decomposing organic matter (saprotrophs). So, the correct answer is B. heterotrophs.