[A]: It is not possible to take photograph and count chromosomes when they are highly coiled and condensed.
[R]: Each species has a variable number of chromosomes.

Options

A. If both A and R are true and R is the correct explanation of A
B. If both A and R are true but R is not the correct explanation of A
C. If A is true and R is false
D. If both A and R are false

Answer

**Step-by-Step Solution:**

1. **Understanding the Assertion (A)**: The assertion states that it is not possible to take photographs and count chromosomes when they are highly coiled and condensed. This is true because during certain phases of cell division (like metaphase), chromosomes are highly condensed and coiled, making them visible under a microscope, but counting them accurately can be challenging.

**Hint**: Think about the structure of chromosomes during different phases of the cell cycle.

2. **Understanding the Reason (R)**: The reason states that each species has a variable number of chromosomes. This is also true. Different species have different chromosome numbers; for example, humans have 46 chromosomes, while fruit flies have 8, and dogs have 78.

**Hint**: Consider examples of different species and their chromosome counts.

3. **Analyzing the Relationship Between A and R**: Although both the assertion and reason are true, they address different aspects. The assertion discusses the visibility and counting of chromosomes when they are coiled, while the reason discusses the variability in chromosome numbers among species. Therefore, R does not explain A.

**Hint**: Determine if the reason directly supports or explains the assertion.

4. **Choosing the Correct Option**: Given that both A and R are true but R is not a correct explanation for A, the correct answer is that both A and R are true, but R is not the correct explanation for A.

**Hint**: Review the answer choices carefully to find the one that matches your conclusion.

**Final Answer**: The correct option is that both A and R are true, but R is not the correct explanation for A.