**Core Concept**
Progesterone is added to estrogen therapy in hormone replacement therapy (HRT) to prevent endometrial hyperplasia and cancer, and to regulate the menstrual cycle. Estrogen alone can stimulate the endometrium, leading to unopposed growth and potential malignancy.

**Why the Correct Answer is Right**
Progesterone counteracts the proliferative effects of estrogen on the endometrium by inducing secretory transformation and preparing the uterus for potential implantation of a fertilized egg. This is achieved through the progesterone receptor-mediated regulation of gene expression, particularly in the endometrial glands and stroma. The combination of estrogen and progesterone in HRT helps to maintain a balanced hormonal environment, reducing the risk of endometrial hyperplasia and cancer.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the effects of adding progesterone to estrogen therapy.
**Option B:** This option is incorrect because it does not provide a clear explanation of the benefits of combining progesterone with estrogen in HRT.

**Clinical Pearl / High-Yield Fact**
A mnemonic to remember the effects of progesterone in HRT is "Protects the Endometrium Regularly, Prevents Hyperplasia, and Promotes a Normal Cycle."

**Correct Answer: C. Prevents endometrial hyperplasia and cancer**