A 50-year-old diabetic patient with asymptomatic gallstone (>3 cm) will be best treated by
Prophylactic cholecystectomy should be conducted in this patient even though it is a silent gallstone with no symptoms as he is a diabetic & stone size is more than 3 cm.
Such patients are at increased risk of complications from gallstones.

Reference: Bailey & Love's short practise of surgery,25 th edition, page no:1121.
Early surgery
Bile salt treatment
ESWL
Waiting till it becomes symptomatic