The passage says that the medication has been shown to be less effective when taken after grapefruit consumption because grapefruit consumption has been shown to affect the binding of the medication to pain receptors. It can be concluded that effective binding is needed to enable the full effectiveness of the medication, which is what (D) states. Regarding (A), the passage says that grapefruit or grapefruit juice ingested within an hour will “significantly” diminish the effectiveness of the medication. This does not mean that grapefruit or grapefruit juice ingested more than an hour before the medication is ingested will have no effect on the medication; it may have a mild effect. Regarding (B), the passage speaks only of taking the medication after ingesting grapefruit; it says nothing about what might happen when eating grapefruit after taking the medication. As for (C), the passage implies that the pain receptors that the specific medication works on are the pain receptors that are affected by grapefruit. This does not mean that these are the only pain receptors in the brain. (E) is out of scope. You are given no information about possible research or design of new migraine medications.