Milk glands are found in

Options

A. All vertebrates
B. All mammals
C. All placental mammals
D. All prototherians only

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks where milk glands, also known as mammary glands, are found.

2. **Defining Mammary Glands**: Mammary glands are specialized organs that produce milk in mammals. The term "mammary" is derived from the Latin word "mama," which means breast.

3. **Identifying the Options**: The options provided are:
- All vertebrates
- All mammals
- All placental mammals
- All prototherians only

4. **Analyzing Each Option**:
- **All Vertebrates**: This option is incorrect because not all vertebrates have mammary glands. Vertebrates include groups like reptiles, amphibians, and birds, which do not possess these glands.
- **All Mammals**: This option is correct. All mammals, including prototherians (like monotremes), metatherians (like marsupials), and eutherians (placental mammals), have mammary glands.
- **All Placental Mammals**: This option is incorrect because it excludes prototherians and metatherians, which also have mammary glands.
- **All Prototherians Only**: This option is also incorrect as it limits the presence of mammary glands to only one group of mammals, while other groups also have them.

5. **Conclusion**: After analyzing all options, the correct answer is **all mammals** since all members of this class possess mammary glands.

### Final Answer:
Milk glands are found in **all mammals**.