Which of the following characteristic features always holds true for the corresponding group of animals?

Options

A. Cartilaginous endoskeleton -Chondrichthyes
B. Viviparous - Mammalia
C. Possess a mouth with an upper and a lower jaw - Chordata
D. 3-chambered heart with one incompletely divided ventricle - Reptilia

Answer

To solve the question regarding the characteristic features that always hold true for the corresponding group of animals, we will analyze each option provided.

### Step-by-Step Solution:

1. **Option 1: Cartilaginous Endoskeleton - Chondrichthyes**
- Chondrichthyes, which includes sharks and rays, are known for having a cartilaginous endoskeleton. This characteristic is consistent across all members of this class, making this option true.

2. **Option 2: Viviparous - Mammalia**
- While most mammals are viviparous (giving birth to live young), there are exceptions. The platypus and echidna are examples of oviparous mammals (they lay eggs). Therefore, this option is not always true.

3. **Option 3: Possesses a Mouth with Upper and Lower Jaw - Chordata**
- While most chordates have a mouth with upper and lower jaws, cyclostomes (like lampreys and hagfish) do not possess jaws. Thus, this characteristic does not hold true for all chordates.

4. **Option 4: 3 Chambered Heart with Incompletely Divided Ventricle - Reptilia**
- Most reptiles do have a 3-chambered heart; however, crocodiles are an exception as they possess a 4-chambered heart. Therefore, this characteristic is not universally applicable to all reptiles.

### Conclusion:
The only option that holds true for all members of the corresponding group of animals is **Option 1: Cartilaginous Endoskeleton - Chondrichthyes**.