This supports the alternative hypothesis that the seeds floated across the ocean. It doesn’t support the biologist’s hypothesis that seabirds carried the seeds to Réunion Island.

This strengthens the biologist's argument by addressing the assumption that seabirds actually live on both Réunion and Hawaii and are likely to carry seeds between the mountains. If mountain-nesting seabirds are common to both islands, his hypothesis becomes much more likely.

The argument is only concerned with the fact that A. heterophylla is descended from A. koa. It doesn’t matter what tree A. koa is descended from.

Like (A), this supports the alternative hypothesis by implying that the seeds could’ve floated from Hawaii to Réunion. Instead, we need an answer choice that strengthens the biologist’s hypothesis that seabirds carried the seeds between the islands.

Irrelevant— we don’t know whether the “many seabirds species” in (E) are living on Réunion or Hawaii. Even if they are, this weakens the argument by suggesting that the seabirds rarely return to land to disperse the seeds.