Final answer:
An abdominal ultrasound is the most appropriate test to diagnose cholecystitis in Mr. Robertson, as it provides direct imaging of the gallbladder.
Step-by-step explanation:
The best test to help decide if Mr. Robertson has cholecystitis would most likely be c. Ultrasound of the abdomen. While liver function tests (LFTs) can indicate evidence of gallbladder disease, and serum amylase and lipase checks can look for pancreatic inflammation, it is the ultrasound that can directly image the gallbladder to detect stones, inflammation, or other abnormalities typically associated with cholecystitis.