This doesn’t weaken the argument because it shows a similarity between H. pylori and M. tuberculosis. Instead, we need an answer choice that shows that the two bacteria are dissimilar in some relevant way.

Even if tuberculosis lasts longer than stomach ulcers, H. pylori still causes harmful stomach ulcers. So, this doesn’t weaken the conclusion that H. pylori shouldn’t be considered a beneficial commensal. It also doesn’t provide a relevant dissimilarity between the bacteria.

(C) weakens the argument by showing that M. tuberculosis and H. pylori are dissimilar in at least one relevant way. Both cause illness in some people who harbor the bacterium, but M. tuberculosis is not beneficial at all, while H. pylori allegedly strengthens immune response.

This is a dissimilarity between the bacteria, but it isn’t relevant. Regardless of how many people harbor each bacterium, they both cause some harm to some people, which is why the author concludes that, like M. tuberculosis, H. pylori shouldn’t be considered a commensal.

Like (D), the number of people who harbor each bacterium is not a relevant dissimilarity.