Which one of the following paris of chemical substances, is correctly categorised ?

Options

A. `{:("(1)""Calcition and thymosin","Thyroid hormones",,):}`
B. `{:("(2)""Pepsin and prolactin","Two digestive enzymes secreted in stomach",,):}`
C. `{:("(3)""Troponin and myosin","Complex proteins in striated muscles",,):}`
D. `{:("(4)""Secretin and rhodopsin","Polypeptide hormones",,):}`

Answer

To solve the question "Which one of the following pairs of chemical substances is correctly categorized?", we need to analyze each option provided and determine if the pairs are correctly matched based on their functions and classifications.

### Step-by-Step Solution:

1. **Analyze Option A: Calcitonin and Thymosin**
- **Calcitonin**: This is a hormone produced by the parafollicular cells of the thyroid gland. Its primary function is to lower blood calcium levels.
- **Thymosin**: This is a hormone produced by the thymus gland, which plays a crucial role in the development and differentiation of T-cells (a type of immune cell).
- **Conclusion**: Both substances are correctly categorized as hormones, but they are from different glands and serve different functions. This option is correctly paired.

2. **Analyze Option B: Pepsin and Prolactin**
- **Pepsin**: This is a digestive enzyme that breaks down proteins in the stomach.
- **Prolactin**: This is a hormone released by the pituitary gland that stimulates milk production in mammals.
- **Conclusion**: This pair is incorrectly categorized because pepsin is an enzyme, while prolactin is a hormone.

3. **Analyze Option C: Troponin and Myosin**
- **Troponin**: This is a complex of proteins found in skeletal and cardiac muscle fibers that regulate muscle contraction.
- **Myosin**: This is a motor protein that interacts with actin to cause muscle contraction.
- **Conclusion**: Both troponin and myosin are proteins involved in muscle contraction, making this pair correctly categorized.

4. **Analyze Option D: Secretin and Rhodopsin**
- **Secretin**: This is a hormone that stimulates the secretion of pancreatic juices and bile.
- **Rhodopsin**: This is a light-sensitive receptor protein involved in visual phototransduction in the retina.
- **Conclusion**: This pair is incorrectly categorized because secretin is a hormone, while rhodopsin is a protein involved in vision.

### Final Answer:
The correctly categorized pair of chemical substances is **Option C: Troponin and Myosin**.