**Core Concept**
Endometrial cancer is a malignancy of the endometrium, the inner lining of the uterus. The histological subtypes of endometrial cancer are characterized by distinct cellular and histological features, which have implications for prognosis and treatment.

**Why the Correct Answer is Right**
The most common histological subtype of endometrial cancer is Endometrioid Adenocarcinoma (Type I). This subtype accounts for approximately 80% of all endometrial cancers and is typically associated with estrogen receptor (ER) positivity, a favorable prognosis, and a high response rate to hormone therapy. The pathogenesis of Endometrioid Adenocarcinoma involves mutations in genes such as PTEN and PIK3CA, which lead to the activation of the PI3K/AKT signaling pathway and the promotion of cell proliferation.

**Why Each Wrong Option is Incorrect**
**Option A:** Serous Carcinoma is a rare but aggressive subtype of endometrial cancer, accounting for approximately 10% of cases. It is often associated with a poor prognosis and a high rate of metastasis.
**Option B:** Clear Cell Carcinoma is a rare subtype of endometrial cancer, accounting for approximately 2-5% of cases. It is often associated with a poor prognosis and a high rate of recurrence.
**Option C:** Mucinous Carcinoma is a rare subtype of endometrial cancer, accounting for approximately 1-2% of cases. It is often associated with a favorable prognosis, but can be more challenging to treat due to its mucinous differentiation.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the histological subtype of endometrial cancer has significant implications for prognosis and treatment. Endometrioid Adenocarcinoma (Type I) is typically managed with hormone therapy, while Serous Carcinoma and Clear Cell Carcinoma often require more aggressive treatment approaches, including surgery and chemotherapy.

**Correct Answer:** C.