A 67-year-old man with alcohol dependence has mild withdrawal 10 hours after his last drink, with anxiety, tremor and nausea but no hallucinations or seizure. He has Child-Pugh C cirrhosis, a previous episode of hepatic encephalopathy and chronic obstructive pulmonary disease; his respiratory rate is 18/min with oxygen saturation 95% on room air. Which one of the following is the most appropriate oral medication for symptom-triggered treatment?