(A) identifies a possibility the author needed to address but didn't. If the high-tendency patients laughed more at the videos, then their greater immune gains might just be a result of more laughter, not some special property of their tendency. We'd have no reason to jump to the author's conclusion that tendency itself helps if the high-tendency group simply laughed more during the study.

Maybe the high-tendency group already had stronger immune systems before the study. But the study measured gains in immune system strength, not absolute levels. Even if one group started with stronger immune systems, what matters is whose immune systems grew more from watching the videos. Starting stronger doesn't explain gaining more.

The author's conclusion is about hospital patients, not the general population. She doesn't generalize beyond the group studied, so there's no need for hospital patients to be representative of everyone.

This suggests that gaining immune strength might have changed patients' tendency to laugh. But the premise describes patients whose tendency to laugh was "greater to begin with" (meaning, before the study began). Their tendency was measured first, then they watched the videos. So the gains couldn't have influenced a tendency that was already established.

The author doesn't assume that the high-tendency group recovered faster. The conclusion and the study are about who recovered more, not who recovered faster. These are different things. You can recover a large amount slowly or a small amount quickly. Speed of recovery isn't what's being measured or concluded.