The answer to this question can be determined by understanding the concept of energy transfer through trophic levels in an ecosystem.

1. Trophic Levels in an Ecosystem:
- An ecosystem is a community of living organisms and their interactions with the abiotic environment.
- Within an ecosystem, organisms are organized into trophic levels based on their feeding relationships.
- The trophic levels include producers, primary consumers, secondary consumers, and top carnivores.

2. Producers:
- Producers, also known as autotrophs, are organisms that can produce their own food through photosynthesis or chemosynthesis.
- They convert sunlight energy or chemical energy into organic compounds that can be utilized by other organisms.
- In a pond ecosystem, examples of producers include algae, aquatic plants, and phytoplankton.

3. Energy Transfer:
- Energy flows through an ecosystem in a unidirectional manner, from producers to consumers.
- As energy is transferred from one trophic level to the next, it decreases, resulting in less energy available for higher trophic levels.
- This is known as the 10% rule, which states that only about 10% of the energy from one trophic level is transferred to the next.

4. Top Carnivores:
- Top carnivores, also known as tertiary consumers, are organisms that occupy the highest trophic level in an ecosystem.
- They feed on other consumers, including secondary consumers and sometimes primary consumers.
- Examples of top carnivores in a pond ecosystem may include large fish, birds of prey, or amphibians.

5. Applying the 10% Rule:
- The 10% rule allows us to estimate the number of top carnivores that can be supported by a given number of producers.
- If there are 10 million producers in the pond ecosystem, only about 10% of the energy produced by them will be transferred to the next trophic level.
- Therefore, the number of primary consumers (10% of 10 million) would be approximately 1 million.
- Similarly, the number of secondary consumers (10% of 1 million) would be approximately 100,000.
- Finally, the number of top carnivores (10% of 100,000) would be approximately 10,000.

6. Conclusion:
- Based on the application of the 10% rule, the number of top carnivores that the pond ecosystem can support is approximately 10,000 (in thousands), which corresponds to 10 million in the given options.
- Therefore, the correct answer is option 'B', which states that the pond ecosystem can support 2 million top carnivores.

But in pond ecosystem, the pyramid of number is inverted right.