To solve the question "Which one of the following pairs of organs includes only the endocrine glands?", we will analyze each option provided in the context of endocrine and exocrine glands.
### Step-by-Step Solution:
1. **Understanding Endocrine and Exocrine Glands**:
- Endocrine glands are ductless glands that secrete hormones directly into the bloodstream. Examples include the pituitary gland, thyroid gland, adrenal glands, and parathyroid glands.
- Exocrine glands have ducts and secrete substances like enzymes or digestive juices into specific areas (e.g., pancreas, salivary glands).
2. **Analyzing the Options**:
- **Option 1: Thymus and Testis**:
- The thymus is an endocrine gland that produces thymosins, which are important for immune function.
- The testis, while it produces testosterone (an endocrine function), also functions as a primary sex organ by producing sperm. Therefore, this pair includes an exocrine function, making it an incorrect option.
- **Option 2: Adrenal and Ovary**:
- The adrenal glands are purely endocrine glands that secrete hormones like adrenaline and cortisol.
- The ovary produces hormones (estrogen and progesterone) but also functions as a primary sex organ by producing ova (eggs). Thus, this option also includes an exocrine function, making it incorrect.
- **Option 3: Parathyroid and Adrenal**:
- The parathyroid glands are purely endocrine glands that secrete parathyroid hormone, which regulates calcium levels in the blood.
- The adrenal glands, as previously mentioned, are also purely endocrine glands. This pair consists solely of endocrine glands, making this option correct.
- **Option 4: Pancreas and Ovary**:
- The pancreas has both endocrine (insulin and glucagon) and exocrine functions (producing digestive enzymes). Therefore, it is not purely an endocrine gland.
- The ovary, as discussed, also has an exocrine function. Thus, this option is incorrect.
3. **Conclusion**:
- The only option that includes only endocrine glands is **Option 3: Parathyroid and Adrenal**.
### Final Answer:
**Option 3: Parathyroid and Adrenal**
