Final answer:
Data from a large-scale clinical trial that supports the doctor's claim would strengthen the medical doctor's argument.
Step-by-step explanation:
The most strengthening piece of evidence for the medical doctor's argument would be B) Data from a large-scale clinical trial that supports the doctor's claim. Anecdotal evidence from a single patient (A) and testimonials from several individuals without scientific validation (C) may be biased and not necessarily representative of the general population. A competing theory proposed by another medical professional (D) may challenge the doctor's argument but does not strengthen it.