According to Darwin, fitness refers to
a) Ability to survive and reproduce
b) High aggressiveness
c) Healthy appearance
d) Physical strength

Options

A. Ability to survive and reproduce
B. High aggressivness
C. Healthy appearance
D. Physical strength

Answer

To solve the question "According to Darwin, fitness refers to," we need to analyze the provided options in the context of Darwin's theory of evolution.

### Step-by-Step Solution:

1. **Understanding Darwin's Concept of Fitness**:
- Darwin's theory of evolution emphasizes that fitness is not solely about physical attributes or appearances. Instead, it focuses on an organism's ability to survive and reproduce in its environment.

2. **Evaluating the Options**:
- **Option a) Ability to survive and reproduce**: This aligns with Darwin's definition of fitness. It encapsulates the idea that an organism's success is measured by its capability to pass on its genes to the next generation.
- **Option b) High aggressiveness**: While aggression can play a role in survival, it does not define fitness on its own. Not all aggressive individuals are successful in reproducing.
- **Option c) Healthy appearance**: A healthy appearance does not guarantee that an organism can survive or reproduce effectively. It might be misleading.
- **Option d) Physical strength**: Similar to aggressiveness, physical strength alone does not determine an organism's overall fitness. An organism can be strong but still fail to reproduce.

3. **Conclusion**:
- Based on the analysis, the best answer is **Option a) Ability to survive and reproduce**. This option encompasses the broader definition of fitness as understood in the context of Darwin's theory.

### Final Answer:
**a) Ability to survive and reproduce**

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