This possibility, if true, provides another explanation for why the [greater tendency to laugh] people recovered more. It could be that they laughed more, and not due to the underlying inherent greater tendency, divorced from the actual amount of laughter.

The possibility in (B) wouldn’t affect the argument, because the study involved a comparison of which kind of patient recovered more. It was about who experienced a greater increase in immune system strength, not about how one group’s absolute strength compared to the other’s.

The author didn’t generalize from hospital patients to the entire population. The conclusion is about hospital patients, not other people.

The author doesn’t assume this, because the premise is about people whose tendency to laugh was “greater to begin with.” So they started with a greater tendency, then watched the funny videos.

The speed with which patients recovered is irrelevant, because both the study and conclusion concern a comparison regarding who recovered “more”. Someone can recover a small amount quickly or slowly, or a large amount quickly or slowly. Rapidity is different from magnitude.