Question

what important idea did darwin develop after observing the organisms on the galápagos islands?
that species can change over time
that organisms on islands and the mainland are the same
that all birds on the islands were identical
that finch beaks are shaped the same

Explanation:

Darwin's key idea from Galápagos observations was that species can change over time (evolution by natural selection). The other options are incorrect: organisms on islands vs mainland differ, birds on islands aren't identical, and finch beaks relate to adaptation (supporting species change).

Answer:

The first option (That species can change over time)