**Core Concept**
The mammary gland is a modified sweat gland that undergoes significant changes during pregnancy to produce milk for lactation. This modification is essential for the nourishment of newborns.

**Why the Correct Answer is Right**
During embryonic development, the mammary gland forms from the ectodermal outgrowths called mammary ridges. These ridges eventually give rise to the mammary glands, which are composed of ducts, alveoli, and a nipple. In females, the mammary glands are modified to produce milk, a complex mixture of proteins, fats, and carbohydrates, through a process called lactation. This modification involves the differentiation of mammary epithelial cells into lactating cells, which synthesize and secrete milk.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the mammary gland is not a modified sebaceous gland. Sebaceous glands produce sebum, an oily substance that lubricates the skin and hair, whereas mammary glands produce milk for lactation.
* **Option B:** This option is incorrect because the mammary gland is not a modified apocrine gland. Apocrine glands are a type of sweat gland found in the armpits and groin area, which produce a type of sweat that contains protein and lipids. Mammary glands, on the other hand, produce milk through a different process.
* **Option C:** This option is incorrect because the mammary gland is not a modified salivary gland. Salivary glands produce saliva, a watery secretion that contains enzymes and other compounds necessary for digestion. Mammary glands produce milk, a complex mixture of proteins, fats, and carbohydrates.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the mammary gland is a modified sweat gland that undergoes significant changes during pregnancy to produce milk for lactation. This modification is unique to females and is essential for the nourishment of newborns.

**Correct Answer:** C. Sweat gland.