D. Autotrophs are the organisms that produce their own food through photosyntesis, which means they use the energy provided by the sun to make inorganic molecules (such as CO2) become organic molecules (such as sugar). This process always releases another inorganic molecule as well, different from the one they used as substrate. Plants release O2 when they produce sugar, that's why we call that oxygenic photosyntesis. On the contrary, when sulfur bacteria produce sugar, they release elemental sulfur instead, so they don't produce O2 - that's why we call that anoxygenic photosyntesis. But both processes use the energy of the sun, that's why we say both organisms are autotrophs.
