Necessary, because if it were not true — if an emotion-provoking situation IS enough to cause nonrepressors’ heart rates to rise sharply — then the premise concerning nonrepressors no longer provides support to the conclusion. There would be an alternate explanation available for the nonrepressors situation, weakening the argument. So the author must assume (A).

Whether each group repressed equally as well or one was better at inhibiting than the other is irrelevant, because we know that both repressors and nonrepressors inhibited emotion. Also, the argument doesn’t specify “facial and bodily” displays of emotion; there’s no reason to think the author must assume anything about specific displays.

Not necessary, because even if both groups are equally excited, that doesn’t undermine the author’s reasoning, which is based on the correlation observed between inhibiting emotions and a sharp heart rate increase.

We don’t know whether any of the nonrepressors were “ordinarily very emotional,” so there’s no reason to think the author must assume that very emotional people can refrain from feeling strong emotions. Also, the nonrepressors were asked to inhibit their display of emotion; that’s different from not feeling emotion.

Not necessary, because even if each group started with different average heart rate, we still know that each group experienced a sharp increase in heart rate. The argument isn’t based on a comparison of the repressors’ heart rate vs. the nonrepressors’ heart rate. It’s based on a comparison, within each group, of their heart rate before inhibiting and their heart rate after inhibiting.