Answer: C. An autotroph uses an external energy source to synthesize carbon compounds from simple inorganic substances. Organism C receives external light energy and its sole carbon-containing input is CO₂, a simple inorganic substance.

A: treats glucose, an organic carbon source, as a simple inorganic substance.
B: treats amino acids, which are organic, as a simple inorganic carbon source.
D: treats starch, an organic carbon source, as a simple inorganic substance.
Remember: autotrophy depends on the source of carbon, not only on the energy source.