To solve the question regarding which organisms possess a completely four-chambered heart, we can break down the information as follows:
### Step-by-Step Solution:
1. **Understanding Heart Structure in Animals**:
- The heart is a vital organ that pumps blood throughout the body. The structure of the heart varies among different classes of animals.
- A four-chambered heart consists of two atria and two ventricles, allowing for complete separation of oxygenated and deoxygenated blood.
2. **Identifying Groups with Four-Chambered Hearts**:
- **Mammals**: All mammals have a completely four-chambered heart. This structure is essential for their high metabolic rates and endothermic (warm-blooded) nature.
- **Birds**: All birds also possess a four-chambered heart. Similar to mammals, this adaptation supports their high energy demands, especially during flight.
- **Reptiles**: Most reptiles have a three-chambered heart (two atria and one ventricle). However, there are exceptions, such as crocodiles, which possess a four-chambered heart.
3. **Evaluating the Options**:
- **Option 1**: All reptiles, all birds, all mammals - Incorrect, as not all reptiles have a four-chambered heart.
- **Option 2**: Some reptiles, all birds, all mammals - Correct, as it includes crocodiles (some reptiles) along with all birds and mammals.
- **Option 3**: No reptiles, all birds, all mammals - Incorrect, as it excludes crocodiles.
- **Option 4**: Some reptiles, some birds, and all mammals - Incorrect, as it does not account for all birds having a four-chambered heart.
4. **Conclusion**:
- The correct answer is **Option 2: Some reptiles, all birds, all mammals**. This option accurately reflects the presence of a completely four-chambered heart in these groups.
