Which term does not refer to the energy source for the organism a. phototroph b. chemotroph c. lithotroph d. autotroph

To determine which term does not refer to an energy source for organisms, let's analyze each option:

1. Phototroph: Organisms that obtain energy from light sources.
2. Chemotroph: Organisms that obtain energy from chemical compounds.
3. Lithotroph: Organisms that obtain energy from inorganic substrates.
4. Autotroph: Organisms that produce their own food, generally using light or chemical energy.

Analysis:

• All the first three terms (phototroph, chemotroph, lithotroph) describe how organisms obtain their energy from external sources.
• "Autotroph," while it describes how organisms produce their own food, does not specifically refer to the source of energy; it classifies organisms based on their ability to synthesize organic compounds.

Conclusion:

Therefore, the option that does not directly refer to an energy source is:

d. autotroph.