Assertion: Cholagogues are substances that cause the contraction of gall bladder.
Reason: These substances cause release of CCK-PZ from duodenum

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that "Cholagogues are substances that cause the contraction of the gall bladder." This means that cholagogues help in the release of bile from the gall bladder into the intestine, which is indeed true.

2. **Understanding the Reason**: The reason provided is that "These substances cause the release of CCK-PZ from the duodenum." CCK-PZ, or cholecystokinin, is a hormone that stimulates the gall bladder to contract and release bile. This statement is also true.

3. **Analyzing the Relationship**: The assertion is about the function of cholagogues in promoting gall bladder contraction, while the reason explains how this occurs through the release of CCK-PZ from the duodenum. Since CCK-PZ plays a crucial role in stimulating the gall bladder to contract, the reason directly supports the assertion.

4. **Conclusion**: Both the assertion and the reason are true, and the reason correctly explains the assertion. Therefore, the correct answer is that both the assertion and reason are true, and the reason is the correct explanation of the assertion.

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