Among the options you provided, the most responsible factor for bringing about the origin of a new species is isolation. Isolation plays a crucial role in speciation, the process by which new species arise. When a population of organisms becomes geographically or reproductively isolated from other populations of the same species, they are no longer able to freely interbreed and exchange genetic material.

Isolation can occur through various mechanisms, such as geographic barriers (like mountains, rivers, or oceans) that physically separate populations, or through behavioral or ecological differences that prevent individuals from successfully mating with one another. Once isolated, different populations may undergo genetic changes independently, leading to the accumulation of genetic differences over time.

While mutation, natural selection, and sexual reproduction are important factors in the evolutionary process, they are not directly responsible for the origin of a new species. Mutations provide the raw material for genetic variation, natural selection acts on existing genetic variation to favor certain traits, and sexual reproduction promotes genetic recombination. However, it is through isolation and subsequent divergence that new species can ultimately arise.