The correct answer is 2 - the staging of cancer. The TNM (Tumor, Nodes, and Metastasis) classification system is a widely used staging system for cancer  (primarily used for solid tumors) to describe the extent and spread of cancer in a standardized way. It is essential for treatment planning, prognosis determination, and research. The TNM system categorizes cancer using three key factors:

T (Tumor): T describes the size and extent of the primary tumor. It typically includes categories such as T0, T1, T2, T3, and T4, with higher numbers indicating larger and more extensive tumors. Additional modifiers may specify specific details about tumor size, invasion depth, and nearby structure involvement.

N (Nodes): N indicates whether cancer has spread to regional lymph nodes. The N stage is expressed with categories like N0 (no regional lymph node involvement), N1, N2, N3 (increased involvement or the number of affected lymph nodes) and sometimes includes additional details about the size and number of affected nodes.

M (Metastasis): M describes whether cancer has spread to distant body parts beyond the regional lymph nodes. M0 indicates no distant metastasis, while M1 indicates the presence of metastatic disease. Sometimes, subcategories are used to specify the locations of distant metastases.