[A]: The tadpoles become giant tadpoles when fed on thiourea.
[R]: Thiourea is an antithyroid substance

Options

A. If both A and R are true and R is the correct explanation of A
B. If both A and R are true but R is not the correct explanation of A
C. If A is true and R is false
D. If both A and R are false

Answer

To solve the assertion and reason question regarding the effect of thiourea on tadpoles, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "tadpoles become giant tadpoles when fed on thiourea." This implies that when tadpoles are exposed to thiourea, they do not undergo normal metamorphosis and instead grow larger in their larval stage.

2. **Understanding the Reason**:
- The reason states that "thiourea is an antithyroid substance." This means that thiourea inhibits the production of thyroxine, a hormone crucial for the metamorphosis of tadpoles into adult frogs.

3. **Role of Thyroxine in Metamorphosis**:
- Thyroxine is essential for the metamorphosis process in amphibians. It promotes the transformation from the larval stage (tadpole) to the adult form. Without adequate levels of thyroxine, tadpoles cannot undergo this transformation.

4. **Effects of Thiourea**:
- Since thiourea inhibits the synthesis of thyroxine, its presence in the diet of tadpoles would prevent them from producing enough thyroxine. Consequently, this leads to a lack of metamorphosis, causing the tadpoles to remain in their larval form and grow larger, hence becoming "giant tadpoles."

5. **Conclusion**:
- Both the assertion and the reason are true. The reason correctly explains why the assertion occurs: the inhibition of thyroxine production by thiourea leads to the lack of metamorphosis in tadpoles, resulting in their growth as giant tadpoles.

### Final Answer:
- Both assertion and reason are true, and the reason is the correct explanation of the assertion.