A farmer is growing a crop regularly in his field. He uses chemical fertilizers, pesticides, organic manure as well as bio-fertilizers. Very close to his field is a factory which emits smoke as a by product. There is also a huge lake in the nearby area.
Consider the following food chain in the same lake.
Aquatic plant `to` Small fish `to` Big fish `to` Birds
Which of the above organisms is likely to show minimum amount of pesticide concentration in them after considerable time?

Options

A. Aquatic plants.
B. Small fish.
C. Big fish.
D. Birds.

Explanation

To determine which organism in the given food chain is likely to show the minimum amount of pesticide concentration after considerable time, we can analyze the food chain and the concept of bio-magnification.

### Step-by-Step Solution:

1. **Understanding the Food Chain**: The food chain presented is:
- Aquatic plants → Small fish → Big fish → Birds

2. **Identifying Pesticide Sources**: The farmer uses pesticides, which can enter the lake through runoff or groundwater. This means that the aquatic plants in the lake will absorb these pesticides from the water.

3. **Bio-magnification Concept**: Bio-magnification refers to the process where the concentration of harmful substances, such as pesticides, increases as you move up the food chain. Organisms at higher trophic levels tend to accumulate more of these substances because they consume multiple organisms from lower levels.

4. **Analyzing Each Organism**:
- **Aquatic Plants**: These are the primary producers and will absorb pesticides directly from the water. However, they are at the base of the food chain and will have the lowest concentration of pesticides.
- **Small Fish**: These fish eat aquatic plants and will accumulate pesticides from the plants they consume, leading to a higher concentration than the plants.
- **Big Fish**: These fish eat small fish, and thus will accumulate even more pesticides from the small fish they consume.
- **Birds**: Birds are at the top of the food chain and will consume big fish. They will have the highest concentration of pesticides due to the accumulation from all the organisms below them.

5. **Conclusion**: Based on the concept of bio-magnification, the organism that is likely to show the minimum amount of pesticide concentration in them after considerable time is the **Aquatic Plants**.

### Final Answer:
**Aquatic Plants**