A 78-year-old male is evaluated for exertional dyspnea. He was previously asymptomatic, but 4 months ago he began having shortness of breath during moderate levels of activity. An echocardiogram shows an ejection fraction (EF) of 65%, moderate aortic stenosis (AS) with a mean gradient of 28 mmHg, and an aortic valve (AV) area of 1.5 cm^2. Which of the following interventions is most appropriate for this patient?
a) Observation with serial echocardiograms
b) Medical therapy with a beta-blocker
c) Referral for aortic valve replacement
d) Initiation of aspirin and statin therapy