The defining features of living beings is

Options

A. They digest their food
B. They can regenerate
C. They reproduce
D. They respond to external stimuli

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks for the defining features of living beings. This means we need to identify characteristics that are universally applicable to all living organisms.

2. **Analyzing the Options**:
- **Option A: They digest their food**: While many living organisms do digest food, not all living beings have this characteristic (e.g., some organisms absorb nutrients directly).
- **Option B: They can regenerate**: Regeneration is a feature seen in some organisms (like starfish), but not all living beings possess this ability.
- **Option C: They reproduce**: Reproduction is a common feature of living beings, but there are exceptions (e.g., mules are sterile and cannot reproduce).
- **Option D: They respond to external stimuli**: This characteristic is universally applicable to all living beings, as all organisms have some form of response to changes in their environment.

3. **Identifying the Correct Answer**: After analyzing the options, it is clear that the most defining feature of living beings is their ability to respond to external stimuli. This includes various forms of stimuli such as physical, chemical, or biological changes.

4. **Conclusion**: Therefore, the correct answer is **Option D: They respond to external stimuli**.