The order check-in step that maintains pharmacy compliance is C.) Marking off items on the packing slip as they're pulled from the bin.
Filing the order invoice appropriately and labeling packages with appropriate ordering stickers are important steps to ensure the accuracy of the order and to avoid confusion. However, these steps do not necessarily ensure compliance with pharmacy regulations.
Pointing out noticeable damage to the delivery driver is also an important step to ensure the integrity of the delivery, but it does not directly maintain compliance with pharmacy regulations.
Marking off items on the packing slip as they're pulled from the bin is a critical step in maintaining compliance because it helps ensure that the right medications get to the right patients and that the medication orders are accurately accounted for. This step ensures that the pharmacy's inventory is correct, and that all patient orders are filled accurately. This step also helps prevent drug diversion and other compliance issues.
Therefore, the answer is C. Marking off items on the packing slip as they're pulled from the bin.