What should always be checked against the MAR?

The correct choice involves checking the prescription label, which aligns with the MAR (Medication Administration Record). The MAR is a crucial document that tracks what medications a patient should receive, including dosage, timing, and frequency. By verifying the prescription label against the MAR, you ensure that the correct medication is administered at the right time and dosage. This process helps prevent medication errors, enhances patient safety, and ensures that the treatment protocol is followed as prescribed.

While other options like patient medical history, medications available in stock, and doctor's notes might be useful in different contexts of patient care, they do not directly relate to the immediate requirement of confirming what medication to administer. Checking the prescription label three times (often referred to as the "three checks" principle in medication administration) reinforces the importance of accuracy in medication delivery, making it the most appropriate option in this scenario.