In an ecological pyramid, energy decreases as you move up the trophic levels. The base of the pyramid typically consists of primary producers (like phytoplankton and bacteria), which convert sunlight into biomass through photosynthesis. These organisms provide the most available energy because they capture energy directly from the sun.
Based on this understanding, the correct answer to the question would be:
b. phytoplankton and bacteria
These organisms form the foundation of the energy pyramid and provide the most energy available to the entire ecosystem.