Assertion: A group of closely related families form an order.
Reason: The families of an order show close resemblance in certain fundamental features and also in evolutionary trends

Answer

**Step-by-Step Solution:**

1. **Understanding the Classification System**:
- The biological classification system is hierarchical, consisting of several levels: Kingdom, Phylum, Class, Order, Family, Genus, and Species. Each level groups organisms based on shared characteristics.

2. **Analyzing the Assertion**:
- The assertion states that "a group of closely related families forms an order." This is accurate as an order is defined as a higher taxonomic category that includes multiple families that share significant similarities.

3. **Analyzing the Reason**:
- The reason given is that "the families of an order show close resemblance in certain fundamental features and also in evolutionary trends." This is also true, as families within an order exhibit similarities in their characteristics and evolutionary history.

4. **Conclusion**:
- Since both the assertion and the reason are true, and the reason provides a correct explanation for the assertion, we conclude that both statements are valid.

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

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