C

In the given pyramid of numbers, the structure suggests a food chain where the number of individuals decreases as you move up the trophic levels.
- **PP (Primary Producers)**: This level is the base, which typically represents the organisms that create energy from sunlight or inorganic substances. In aquatic ecosystems, phytoplankton are a major component. So, having 'phytoplankton' in level PP is appropriate.
- **SC (Secondary Consumers)**: This level consists of organisms that eat the primary consumers. Small insectivorous birds could be an example if the primary consumers were insects, but considering that phytoplankton is in the bottom level, we can assume the next level should consist of organisms that consume primary producers.
- **TC (Tertiary Consumers)**: At the top level, we usually have larger predators. Whales are known to be atop the marine food chain, which fits well here.
Therefore, option C is coherent with the structure of the pyramid and the ecological roles of the organisms at each level.