Which of the following characteristic features always holds true for the corresponding group of animals
`{:("(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"):}`

Options

A. `{:("Viviparous "," Mammalia "):}`
B. `{:("Possess a mouth"," Chordata"),("with an upper and a ",""),("lower jaw",""):}`
C. `{:("3- chambered heart"," Reptilia"),("with one incompletely",""),("divided ventricle",""):}`
D. `{:("Cartilaginous"," Chondrichthyes"),("endoskeleton",""):}`

Answer

To solve the question regarding which characteristic feature always holds true for the corresponding group of animals, we will analyze each option step by step.

### Step-by-Step Solution:

1. **Option (a): Cartilaginous endoskeleton - Chondrichthyes**
- Chondrichthyes refers to cartilaginous fish, which include sharks and rays. They possess a skeleton made entirely of cartilage rather than bone.
- **Conclusion**: This characteristic is true for all members of Chondrichthyes.

2. **Option (b): Viviparous - Mammalia**
- Viviparous means giving birth to live young rather than laying eggs. While most mammals are viviparous, there are exceptions like the monotreme, specifically the platypus, which lays eggs.
- **Conclusion**: This characteristic does not hold true for all mammals due to exceptions.

3. **Option (c): Possess a mouth with an upper and a lower jaw - Chordata**
- Chordates generally have a mouth with an upper and lower jaw. However, there are exceptions, such as jawless fish (e.g., lampreys and hagfish) that belong to the group Agnatha.
- **Conclusion**: This characteristic does not hold true for all Chordates due to exceptions.

4. **Option (d): 3-chambered heart with one incompletely divided ventricle - Reptilia**
- Most reptiles have a 3-chambered heart, consisting of two atria and one ventricle that is partially divided. However, crocodiles are reptiles that have a 4-chambered heart.
- **Conclusion**: This characteristic does not hold true for all reptiles due to exceptions.

### Final Answer:
The only characteristic feature that always holds true for the corresponding group of animals is **(a) Cartilaginous endoskeleton - Chondrichthyes**.