The correct answer is option 3: Biopsy. A biopsy is a medical procedure where a sample of tissue is taken from the body and examined under a microscope to determine if cancer cells are present. This is the most reliable test for diagnosing cancer as it provides direct evidence of the presence of cancer cells.

Although urine and blood tests can be helpful in detecting certain markers or substances that indicate the possibility of cancer, they are not definitive for diagnosing cancer. These tests can only provide preliminary indications and further testing is usually required.

X-ray is a diagnostic imaging test that can be useful in detecting certain types of cancer, but it cannot definitively diagnose cancer on its own. It is often used in combination with other tests to provide a more complete picture.

Therefore, while urine test, blood test, and X-ray can provide useful information, it is the biopsy test that is essential for confirming a diagnosis of cancer.