A 52-year-old man with a 15-year history of COPD presents to the outpatient clinic with persistent dyspnea on exertion despite being on inhaled salbutamol as needed. His FEV₁ is 45% predicted. On examination, he has mild peripheral edema and elevated JVP. Chest X-ray shows hyperinflation. His current medications include salbutamol inhaler (2–3 times daily) and occasional oral theophylline. The physician decides to add a long-acting bronchodilator to his regimen. Which of the following is the most appropriate choice for maintenance therapy in this patient?