Assertion: Oxytocin is called 'milk-ejection hormone'
Reason: Oxytocin acts on the smooth muscles of uterus and stimulates its contraction. \

Explanation

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that "Oxytocin is called the 'milk-ejection hormone'." This is true because oxytocin plays a crucial role in the process of milk ejection during breastfeeding. When a baby suckles, oxytocin is released, which causes the mammary glands to contract and release milk.

2. **Understanding the Reason**: The reason given is that "Oxytocin acts on the smooth muscles of the uterus and stimulates its contraction." This statement is also true. Oxytocin is known to stimulate contractions of the uterine smooth muscles during childbirth, facilitating labor.

3. **Analyzing the Relationship**: Although both the assertion and the reason are true, we need to determine if the reason correctly explains the assertion. The assertion relates to milk ejection, while the reason pertains to uterine contractions. The action of oxytocin on the uterus does not explain why it is called the 'milk-ejection hormone.'

4. **Conclusion**: Since both statements are true but the reason does not provide a correct explanation for the assertion, we conclude that the correct answer is that both the assertion and reason are true, but the reason is not the correct explanation for the assertion.

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