**Core Concept**
The molecular classification of breast cancer is based on the genetic alterations that occur in the tumor cells. This classification helps in understanding the tumor behavior, predicting the prognosis, and guiding the treatment. The main molecular subtypes of breast cancer are Luminal A, Luminal B, HER2-positive, and Triple-negative.

**Why the Correct Answer is Right**
The molecular classification of breast cancer is primarily based on the estrogen receptor (ER) and progesterone receptor (PR) status, along with the human epidermal growth factor receptor 2 (HER2) status. The ER and PR status determines the Luminal A and Luminal B subtypes, while the HER2 status determines the HER2-positive subtype. The tumors that are negative for ER, PR, and HER2 are classified as Triple-negative. The molecular classification is critical in determining the treatment approach, as Luminal A and Luminal B tumors are generally responsive to hormone therapy, while HER2-positive tumors are treated with HER2-targeted therapy, and Triple-negative tumors are often treated with chemotherapy.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not a valid answer as the molecular classification of breast cancer is not based on the tumor size or location.
**Option B:** This option is not a correct answer as the molecular classification is not based on the tumor grade alone.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the molecular classification of breast cancer is critical in determining the treatment approach and predicting the prognosis. The correct classification helps in personalizing the treatment and improving the patient outcomes.

**Correct Answer: C. Molecular classification of breast cancer is based on the ER, PR, and HER2 status.**