Final answer:
A sore that does not heal may be an early warning sign of cancer, particularly skin cancer, and should be checked by a doctor.
Step-by-step explanation:
The early warning sign of cancer mentioned in the options is C) A sore that does not heal. A sore that does not heal may be a sign of skin cancer, particularly if it is on areas of the skin that are exposed to the sun. Skin cancer is often curable if detected and treated early, so it is important to see a doctor if you notice a sore that does not heal.