Final answer:
An improved level of consciousness indicates that mannitol is having a therapeutic effect on a patient with a closed head injury by reducing intracranial pressure.
Step-by-step explanation:
A nurse assessing a client with a closed head injury and on mannitol would determine that the medication is having a therapeutic effect by noting an improved level of consciousness. Mannitol is an osmotic diuretic used in cases of increased intracranial pressure (ICP) due to its ability to draw extra fluid out of the brain tissue and reduce swelling. By doing so, it can lead to a decrease in ICP, which would be indirectly observed through the improvement of the client's neurological status, reflected in their level of consciousness. A decreased urine output, increased intracranial pressure, or increased blood pressure would represent a lack of therapeutic effect or a worsening of the client's condition. Therefore, mannitol's effectiveness is shown best through neurologic improvement, which aligns with answer option d) Improved level of consciousness.