Solar energy powers Earth's biological systems

Steps:

• Identify the question: seeking the primary energy source for all life processes in ecosystems.
• Recall energy flow: energy enters via sunlight, captured by producers through photosynthesis.
• Trace dependencies: consumers rely on producers, forming food chains, but all originate from solar input.
• Eliminate intermediates: food chains and producers are pathways, not the source; photosynthesis converts solar energy.

Why D is correct:

• The Sun provides radiant energy, which, per the laws of thermodynamics, is the ultimate input fueling photosynthesis and sustaining all trophic levels in biological systems.

Why the others are wrong:

• A: Food chains transfer energy but do not input it; they depend on an external source.
• B: Photosynthesis captures energy but is not the source; it relies on sunlight.
• C: Producers convert energy via photosynthesis but are not the origin; they need solar input.

Final answer: D