Irrelevant. It would be useful to know the rates of ulcers in the physician's country compared to the other two countries. But knowing the rates of those two countries on their own, or compared to each other, doesn't help strengthen the argument..

This weakens the argument. It provides an alternate explanation for why the physician's country tends to prescribe less ulcer medication. People in the physician's country might still get ulcers, but ignore them rather than seeking medication.

Irrelevant. We're not interested in other countries beyond the three discussed here. The physician is drawing a comparison only between her country and the two other countries.

This strengthens the argument. It rules out the possibility that people in the physician's country might still get as many ulcers as people in other countries, but are just less likely to seek medication for cultural or economic reasons.

With more information, this could either strengthen or weaken the physician’s argument. If the other countries tended to over-report prescriptions, then the physician's argument would be weakened. If they tended to under-report, the argument would be strengthened. On its own, though, this answer choice doesn't currently strengthen the argument.