**Mammary Glands in Eutherians**

Mammary glands are specialized glands that are responsible for producing milk in mammals. These glands are unique to mammals and are a defining characteristic of the class Mammalia. In eutherian mammals, which includes most mammals including humans, the mammary glands undergo certain modifications to enable them to produce and secrete milk.

**Modification of Sweat Glands**

The correct answer to the question is option B, sweat glands. Mammary glands in eutherians are derived from the embryonic sweat glands. During development, certain modifications occur in the sweat glands to transform them into mammary glands. This modification involves changes in the structure and function of the glands to produce milk instead of sweat.

**Explanation**

The mammary glands are a type of exocrine gland, meaning they secrete their products, in this case, milk, through a duct. Sweat glands are also exocrine glands that secrete sweat onto the surface of the skin through sweat ducts. During the evolution of mammals, some of the sweat glands underwent modifications to produce milk instead of sweat. This modification allowed for the nourishment of offspring through lactation.

**Importance of Mammary Glands**

Mammary glands play a crucial role in the reproductive success and survival of mammalian species. The ability to produce milk allows mammalian mothers to provide essential nutrients, antibodies, and other bioactive compounds to their offspring, ensuring their growth and development.

**Conclusion**

In summary, mammary glands in eutherian mammals are modifications of sweat glands. These modifications enable the glands to produce and secrete milk instead of sweat. This adaptation is essential for the survival and reproductive success of mammals as it allows for the nourishment and care of their offspring.