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Prostate-specific antigen (PSA) is a well-known example of a tumor marker. Tumor markers are substances that can be found in the blood, urine, or tissues which may indicate the presence of cancer in the body. PSA is used primarily to screen for and monitor prostate cancer. Elevated levels of PSA can suggest prostate cancer or other conditions affecting the prostate, but it is especially valuable in the context of prostate cancer diagnosis and management.

Other options may serve different clinical purposes but do not specifically act as tumor markers for cancer diagnosis. C-reactive protein is an inflammatory marker that indicates systemic inflammation and is not specific to cancer. Hemoglobin A1c is used to monitor long-term glucose control in individuals with diabetes, while thyroid-stimulating hormone plays a role in assessing thyroid function, neither of which provide information about cancer presence. Thus, PSA's direct link to prostate cancer makes it the correct choice as a tumor marker.