Final answer:
Microbes can fix carbon in anoxic environments by chemolithoautotrophy in the absence of light, utilizing inorganic molecules as energy sources for chemosynthesis. Therefore, the correct option is B
Step-by-step explanation:
The microbes can fix carbon in anoxic environments using anoxygenic photosynthesis as well as by chemolithoautotrophy in the absence of light. This process involves organisms known as chemolithotrophs, which are a type of chemoautotroph that derives energy from the oxidation of inorganic molecules. These organisms are vital to ecosystems such as those around hydrothermal vents deep in the ocean, where no light penetrates. They use inorganic molecules like hydrogen sulfide as an energy source for the chemosynthesis of organic compounds, forming the basis of an ecosystem that supports a variety of other organisms.