The common ancestor of all the modern reptiles, birds and dinosaurs is:

Options

A. Synapsids
B. Sauropsids
C. Therapsids
D. Pelycosaurs

Answer

To solve the question regarding the common ancestor of all modern reptiles, birds, and dinosaurs, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Groups Involved**: Identify the groups mentioned in the question: modern reptiles, birds, and dinosaurs. All of these groups are part of the larger clade known as Archosauria.

2. **Identify Common Characteristics**: Note that all these groups share certain characteristics, such as laying eggs. This is a significant trait that links them together.

3. **Examine the Options**: The options given are synapsids, sauropsids, theropods, and pelycosaurs. We need to determine which of these represents the common ancestor.

4. **Define the Terms**:
- **Synapsids**: These are a group of animals that include mammals and their ancestors. They do not include reptiles or birds.
- **Sauropsids**: This group includes reptiles and birds. It is the correct clade that encompasses the common ancestor of modern reptiles, birds, and dinosaurs.
- **Theropods**: These are a subgroup of dinosaurs that include modern birds but do not represent the common ancestor of all reptiles.
- **Pelycosaurs**: These are early synapsids and are not related to reptiles or birds.

5. **Conclude with the Correct Answer**: Based on the definitions and the relationships, the common ancestor of all modern reptiles, birds, and dinosaurs is **sauropsids**.

### Final Answer:
The common ancestor of all the modern reptiles, birds, and dinosaurs is **sauropsids**.

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