Final answer:
The approximate temperature used for heel warming is 42°C, which aids in blood flow for more efficient blood collection without causing harm to the patient.
Step-by-step explanation:
The approximate temperature used for heel warming is 42°C (Option C). Heel warming is a procedure often used in healthcare settings to facilitate blood flow in the heel, making it easier to obtain blood samples, especially from infants. Applying a warm compress to the heel slightly elevates the local temperature, which in turn dilates the blood vessels and aids in easier and more efficient blood collection. The temperature used should be warm enough to allow for vasodilation without causing discomfort or burns to the patient's skin.
Hence, 42°C is used as it is above body temperature but below the threshold that may cause harm. It is important to note that excessive heat application can be dangerous, as noted in the provided reference where an unreasonable temperature rise to 73°C would be harmful.