**B. Producer**

A green plant is classified as a producer in an ecosystem. Let's explore why this is the correct answer in detail:

**1. Producers in an Ecosystem:**
- Producers are organisms that have the ability to produce their own food using energy from the sun through a process called photosynthesis.
- They are the foundation of an ecosystem as they convert sunlight, carbon dioxide, and water into glucose (a type of sugar) and oxygen.
- By producing food, producers provide energy and nutrients to other organisms in the ecosystem.

**2. Photosynthesis:**
- Green plants have a pigment called chlorophyll, which gives them their characteristic green color.
- Chlorophyll helps plants capture sunlight and convert it into energy through photosynthesis.
- During photosynthesis, plants use sunlight, carbon dioxide from the air, and water from the soil to produce glucose and release oxygen as a byproduct.

**3. Role of Green Plants:**
- Green plants are the primary producers in most ecosystems, as they initiate the flow of energy and nutrients.
- They form the basis of the food chain, as they are the primary source of food for other organisms.
- Other organisms, such as herbivores (plant-eating animals) and omnivores (animals that eat both plants and other animals), rely on green plants for their energy and nutrients.

**4. Importance of Green Plants:**
- Green plants play a crucial role in maintaining the balance of ecosystems.
- They absorb carbon dioxide, a greenhouse gas, from the atmosphere, helping to regulate climate.
- Through photosynthesis, they release oxygen, which is essential for the survival of most organisms.
- Green plants also provide habitats and shelter for many organisms.

In conclusion, a green plant is a producer in an ecosystem. It uses sunlight, carbon dioxide, and water to produce glucose and release oxygen through the process of photosynthesis. As primary producers, green plants form the foundation of the food chain and provide energy and nutrients to other organisms in the ecosystem.

Answer is b