Choose the correct statement:

Options

A. All reptiles have a three-chambered heart.
B. All pisces have gills covered by an operculum.
C. All mammals are viviparous.
D. All cyclostomes do not possess jaws and paired fins.

Answer

To solve the question "Choose the correct statement," we will analyze each of the provided statements regarding reptiles, Pisces (fish), mammals, and cyclostomes.

### Step-by-Step Solution:

1. **Analyze the First Statement: "All reptiles have a three-chambered heart."**
- Most reptiles do have a three-chambered heart, which consists of two atria and one ventricle. However, crocodiles are an exception as they possess a four-chambered heart. Therefore, this statement is **incorrect**.

2. **Analyze the Second Statement: "All Pisces have gills covered by an operculum."**
- Pisces (fish) are divided into two groups: bony fish (which have an operculum covering their gills) and cartilaginous fish (like sharks and rays, which do not have an operculum). Since not all fish have an operculum, this statement is also **incorrect**.

3. **Analyze the Third Statement: "All mammals are viviparous."**
- While most mammals are viviparous (giving birth to live young), there are exceptions. For example, the monotreme known as the platypus lays eggs (oviparous). Thus, this statement is **incorrect**.

4. **Analyze the Fourth Statement: "All cyclostomes do not possess jaws and beard fins."**
- Cyclostomes, which include species like lampreys and hagfish, are characterized by the absence of jaws and paired fins. This statement is **correct**.

### Conclusion:
The correct statement among the four options provided is: **"All cyclostomes do not possess jaws and beard fins."**