Regardless of how many of the warblers had reached breeding age, we know the majority of the healthy warblers transplanted to the island maintained a pattern of cooperative breeding. (A) doesn’t help explain this phenomenon.

We don’t know if climate affects warblers’ breeding habits. (B) gives us information that doesn’t aid our understanding of the phenomenon in the stimulus.

This identifies an environmental factor on the neighboring island that could affect warbler breeding habits. If most of the terrain on the new island was not of the type warblers typically use for their nests, it may have caused many to maintain a pattern of cooperative breeding.

We don’t know if the cooperative breeding habits of other species are indicative of the breeding habits of Seychelles warblers. (D) is irrelevant.

This may compound the phenomenon, as less competition for nesting could provide warblers with more space to raise offspring. The transplanted warblers having fewer nesting competitors isn’t an explanation for why many of those warblers maintained a cooperative breeding pattern.