Assertion: Thyroid hormones promote physical growth and development of mental faculties.
Reason: Hypothyroidism in adults causes retarded sexual development.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Assertion:**
- The assertion states that "Thyroid hormones promote physical growth and development of mental faculties." This is true because thyroid hormones, particularly thyroxine (T4) and triiodothyronine (T3), are crucial for normal growth and development. They stimulate protein synthesis and play a significant role in brain development and function.

2. **Understanding the Reason:**
- The reason provided is "Hypothyroidism in adults causes retarded sexual development." This is also true. Hypothyroidism, which is a condition characterized by insufficient production of thyroid hormones, can lead to various physiological issues, including delayed or retarded sexual development in adults.

3. **Analyzing the Relationship:**
- While both the assertion and the reason are true, they address different aspects of thyroid hormone function. The assertion focuses on the general role of thyroid hormones in growth and mental development, while the reason specifically discusses the effects of hypothyroidism on sexual development.

4. **Conclusion:**
- Since both statements are true but the reason does not directly explain the assertion, we conclude that the correct answer is: "Both assertion and reason are true, but the reason is not the correct explanation for the assertion."

**Final Answer:**
- The correct option is B: Both assertion and reason are true, but the reason is not the correct explanation for the assertion.

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