Among living organisms, which one of the following is the most responsible factor for bringing about the origin of a new species?

Speciation fundamentally requires a cessation or strong reduction of gene flow between populations so they can evolve independently; thus isolation (geographic or reproductive) is the proximate factor that allows divergence to proceed. Once isolated, populations accumulate genetic differences through mutation, genetic drift, and natural selection, and evolve pre‑ or post‑zygotic barriers that complete the origin of a new species [1]. Therefore, while mutation and natural selection provide the raw variation and differential survival that drive divergence, isolation is the key condition that permits those processes to produce distinct species by preventing homogenizing gene flow.

Sources

1. [1] https://www.nature.com/scitable/knowledge/library/speciation-the-origin-of-new-species-26230527/