**Core Concept**
The placenta secretes a hormone that stimulates ductal growth in the mammary gland during pregnancy, preparing the breast for milk production. This hormone is a key player in the development of the mammary gland and is essential for lactation.

**Why the Correct Answer is Right**
During pregnancy, the placenta secretes human placental lactogen (hPL), which stimulates the growth and development of the mammary gland. hPL acts on the mammary gland by promoting the growth of ducts and alveoli, which are essential for milk production. This process is mediated by the estrogen and progesterone receptors in the mammary gland, which are activated by hPL.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while estrogen and progesterone are essential for mammary gland development, they are not the primary hormones secreted by the placenta that stimulate ductal growth.
**Option B:** This option is incorrect because prolactin is a hormone that stimulates milk production, but it is not the hormone secreted by the placenta that stimulates ductal growth in the mammary gland.
**Option C:** This option is incorrect because progesterone is essential for maintaining pregnancy, but it is not the primary hormone secreted by the placenta that stimulates ductal growth in the mammary gland.

**Clinical Pearl / High-Yield Fact**
The hormone human placental lactogen (hPL) also has a role in suppressing maternal glucose levels by stimulating insulin resistance, which helps to provide a constant glucose supply to the fetus.

**Correct Answer: C. Progesterone.**