**Core Concept**
The clinical staging of breast cancer involves assessing the extent of tumor invasion (T), lymph node involvement (N), and distant metastasis (M). The T stage is crucial in determining the prognosis and guiding treatment.

**Why the Correct Answer is Right**
The T stage is determined by the size and extent of the primary tumor. In this case, the patient presents with breast heaviness, which suggests a palpable mass. The T stage is typically classified as follows: T1 (≤2 cm), T2 (2.1-5 cm), T3 (5.1-10 cm), and T4 (any size with skin involvement or fixation to the chest wall). The correct answer will depend on the size and characteristics of the tumor.

**Why Each Wrong Option is Incorrect**
**Option A:** T1 is incorrect because the tumor size is not specified, but it is likely larger than 2 cm given the patient's symptoms.
**Option B:** T3 is incorrect because the tumor would need to be significantly larger than 5 cm to be classified as T3.
**Option C:** T4 is incorrect because there is no indication of skin involvement or fixation to the chest wall.

**Clinical Pearl / High-Yield Fact**
When assessing the T stage of breast cancer, it is essential to consider the size and extent of the primary tumor, as well as any evidence of skin involvement or fixation to the chest wall.

**Correct Answer:** .