(D) 99%
Green plants capture only about 1% of the solar energy falling on their leaves and convert it into food energy. Therefore, the percentage not converted is approximately 99%.
Source: Chapter 13, Section 13.1.1 (Food Chains and Webs)
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The passage clearly states: "The green plants in a terrestrial ecosystem capture about 1% of the energy of sunlight that falls on their leaves and convert it into food energy." The question asks for what is not converted, so 100% − 1% = 99%. Students often confuse this with the 10% law (energy transfer between trophic levels), so keep the two values distinct: 1% = solar energy captured by plants; 10% = energy passed from one trophic level to the next.