Aquatic animals are mostly ammonotelic because
(a) Excretion of ammonia requires large amount of water which is available to these animals
(b) Ammonia helps in checking the inflow of water into body
(c) They get lesser light
(d) Water contains less nitrogen

Options

A. Excretion of ammonia requires large amount of water which is available to these animals
B. Ammonia helps in checking the inflow of water into body
C. They get lesser light
D. Water contains less nitrogen

Answer

To solve the question regarding why aquatic animals are mostly ammonotelic, we can analyze each option step by step.

### Step-by-Step Solution:

1. **Understanding Ammonotelism**:
- Ammonotelism is the process by which certain organisms excrete ammonia as their primary nitrogenous waste. Aquatic animals, such as fish, are typically ammonotelic.

2. **Analyzing Option A**:
- This option states that "excretion of ammonia requires a large amount of water which is available to these animals."
- Ammonia is highly soluble in water and requires a significant amount of water for its excretion. Since aquatic animals live in water, they have access to the necessary resources to excrete ammonia efficiently.

3. **Analyzing Option B**:
- This option claims that "ammonia helps in checking the inflow of water into the body."
- This statement is misleading. Ammonia does not play a role in regulating water inflow; rather, it is a waste product that needs to be eliminated.

4. **Analyzing Option C**:
- This option states that "they get lesser light."
- The amount of light an aquatic animal receives does not influence its nitrogenous waste excretion method. This option is irrelevant to the question.

5. **Analyzing Option D**:
- This option claims that "water contains less nitrogen."
- The nitrogen content of water is not a factor that determines whether an animal is ammonotelic. The ability to excrete ammonia is more about the animal's habitat and the solubility of ammonia in water.

6. **Conclusion**:
- After analyzing all options, it is clear that option A is the correct answer. Aquatic animals are mostly ammonotelic because they have access to large amounts of water, which is essential for the excretion of ammonia.

### Final Answer:
(a) Excretion of ammonia requires large amount of water which is available to these animals.

---