The most unique mammalian characteristic is/are

Options

A. The presence of milk producing glands (mammary glands)
B. Heart is completely four chambered
C. The presence of hair on skin
D. Both A and C

Explanation

To determine the most unique mammalian characteristic, we need to analyze each option provided in the question:

1. **Presence of milk-producing glands**: This characteristic is indeed unique to mammals. All mammals possess mammary glands that produce milk to nourish their young. This feature sets mammals apart from other animal groups.

2. **Heart is completely four-chambered**: While it is true that mammals have a four-chambered heart, this characteristic is not exclusive to them. Birds and some reptiles, like crocodiles, also possess a four-chambered heart. Therefore, this feature cannot be considered uniquely mammalian.

3. **Presence of hair on skin**: Hair is another characteristic that is unique to mammals. Other animal groups have different types of body coverings, such as scales in reptiles, feathers in birds, and moist skin in amphibians. The presence of hair is a defining trait of mammals.

4. **Both A and C**: Since both the presence of milk-producing glands and the presence of hair on the skin are unique to mammals, this option combines the two unique characteristics.

Based on this analysis, the correct answer is **Option 4: Both A and C**.

### Step-by-Step Solution:
1. Analyze the first option: Presence of milk-producing glands is unique to mammals.
2. Analyze the second option: Four-chambered heart is not exclusive to mammals as birds and some reptiles also have it.
3. Analyze the third option: Presence of hair on skin is unique to mammals.
4. Combine the unique characteristics from options A and C, leading to the conclusion that both are uniquely mammalian traits.
5. Therefore, the correct answer is Option 4: Both A and C.