To answer the question "Tetrapods include," we need to understand what tetrapods are and analyze the provided options step by step.
### Step-by-Step Solution:
1. **Definition of Tetrapods**:
- Tetrapods are vertebrates that possess four limbs. The term "tetrapod" comes from the Greek words "tetra," meaning four, and "pous," meaning foot. This group includes all animals that have evolved from a common ancestor with four limbs.
2. **Identifying Groups Included in Tetrapods**:
- Tetrapods encompass several classes of animals, including:
- **Amphibia**: This class includes frogs, toads, and salamanders. All amphibians have four limbs in their adult form, making them tetrapods.
- **Reptilia**: This class includes reptiles such as lizards, snakes, and turtles. Most reptiles have four limbs, although snakes are an exception as they have lost their limbs through evolution.
- **Aves**: Birds are also considered tetrapods because their wings are modified forelimbs. Therefore, all birds fall under the category of tetrapods.
- **Mammalia**: This class includes all mammals, such as humans, dogs, and whales. Mammals also possess four limbs, categorizing them as tetrapods.
3. **Analyzing the Options**:
- **Option A**: Amphibia, Reptilia, Aves, and Mammalia. This option includes all the major groups of tetrapods.
- **Option B**: Reptilia, Mammalia, and Amphibia. This option is missing Aves (birds).
- **Option C**: Amphibia and Reptilia. This option is missing both Aves and Mammalia.
- **Option D**: Apes and Mammalia. This option is too narrow as it only includes a subset of mammals and misses other tetrapod classes.
4. **Conclusion**:
- The only option that includes all major groups of tetrapods is **Option A**: Amphibia, Reptilia, Aves, and Mammalia. Therefore, the correct answer is Option A.
### Final Answer:
**Tetrapods include: Amphibia, Reptilia, Aves, and Mammalia (Option A).**
