The productivity of an ecosystem refers to the rate at which energy is converted into organic matter by organisms through the process of photosynthesis or feeding. In this context, the productivity of different trophic levels can be compared to determine which group has the lowest productivity.

1. Trophic Levels:
- Quaternary Consumers: These are organisms that occupy the highest trophic level in the food chain or web. They feed on tertiary consumers.
- Herbivores: These are organisms that consume plant material as their primary source of energy.
- Carnivores: These are organisms that primarily consume other animals as their source of energy.
- Top Carnivores: These are the apex predators at the highest trophic level which feed on other carnivores.

2. Explanation:
The productivity of an ecosystem decreases as we move up the trophic levels due to energy loss at each level. This is known as the 10% rule, where only around 10% of the energy is transferred from one trophic level to the next. The rest is lost as heat or used for metabolic processes.

- Quaternary Consumers: Due to the energy loss at each trophic level, the quaternary consumers receive the least amount of energy compared to the other groups. They depend on tertiary consumers for their energy, which in turn depend on secondary consumers, and so on. As a result, the productivity of quaternary consumers is the lowest among the given options.
- Herbivores: Herbivores obtain energy directly from plants, which are the primary producers. Although they also experience energy loss, it is relatively lower compared to the quaternary consumers since they are one trophic level closer to the primary source of energy.
- Carnivores: Carnivores obtain energy by consuming other animals. They are one trophic level higher than herbivores and thus experience more energy loss. However, their productivity is higher than quaternary consumers as they are closer to the primary source of energy.
- Top Carnivores: Top carnivores, being at the highest trophic level, experience the highest energy loss. However, their productivity is still higher than quaternary consumers as they are closer to the primary source of energy.

In conclusion, the productivity of quaternary consumers is the lowest among the given options due to the significant energy loss at each trophic level.