Explanation:

Ecosystems are composed of various functional units that work together to maintain the balance of the ecosystem. These functional units include productivity, energy flow, and decomposition. However, stratification is not considered a functional unit of an ecosystem.

Productivity:
- Productivity refers to the rate at which energy is converted into biomass by primary producers through the process of photosynthesis.
- Primary producers, such as plants and algae, capture sunlight energy and convert it into organic compounds, which then serve as food for other organisms in the ecosystem.
- Productivity is an essential functional unit as it determines the amount of energy available in the ecosystem and supports the growth and survival of other organisms.

Energy Flow:
- Energy flow refers to the transfer of energy through different trophic levels in an ecosystem.
- Energy enters the ecosystem through the primary producers and then moves from one trophic level to another as organisms consume each other.
- Energy flow is crucial for the functioning of an ecosystem as it determines the availability of energy for various biological processes.

Decomposition:
- Decomposition is the breakdown of complex organic matter into simpler inorganic compounds by decomposers, such as bacteria and fungi.
- Decomposers play a vital role in recycling nutrients by breaking down dead organisms and organic waste, releasing nutrients back into the ecosystem.
- Decomposition ensures the continuous availability of essential nutrients for the growth of primary producers and other organisms in the ecosystem.

Stratification:
- Stratification refers to the division of an ecosystem into different layers based on physical and ecological factors.
- It is commonly observed in aquatic ecosystems where different species occupy different layers based on their adaptations and habitat requirements.
- Stratification is not considered a functional unit of an ecosystem because it does not directly contribute to the energy flow or nutrient cycling processes.
- While it may influence the distribution and interactions of organisms within an ecosystem, it is not an essential component for the functioning of the ecosystem.

Therefore, among the given options, stratification is not a functional unit of an ecosystem.

Refer ncert