Final answer:
The purple non-sulfur bacterium Rhodospirillum grows best with light as its energy source and organic compounds like fructose as its carbon source, so the correct answer is C) fructose and light.
Step-by-step explanation:
The most favourable sources of energy and carbon for the purple non-sulfur bacterium Rhodospirillum, which grows best as a photoheterotroph, are light and organic carbon sources. Therefore, among the given options, the correct answer is C) fructose and light. As photoheterotrophs, purple non-sulfur bacteria like Rhodospirillum capture light energy to convert to chemical energy in the cells but obtain their carbon from organic compounds. They are known to use organic substrates such as succinate and malate as electron donors. This distinguishes them from photoautotrophs, which use CO₂ as their carbon source, and from organisms that depend on inorganic sources such as methane.