I Believe The Answer Is B. Physical Weathering.
The mechanisms of reproductive isolation prevent the reproduction of two individuals from different species. The prezygotic mechanisms disrupt the stages of reproduction before the forming of the zygote or prevent the mating at the start, and the postzygotic mechanisms affect the stages of reproduction after the forming of the zygote.
So, the first and the third example (the urchins and the grasshoppers) show the prezygotic mechanism, as the two individuals are not able to mate or form a zygote.
The second and the fourth example ( zonkey and the death of a zygote) show the working of the postzygotic mechanisms, as the zygote is formed, but it seems to be inviable, and the zonkeys are sterile, preventing the individual to reproduce.
A derived character refers to a particular character that is shared by members of a particular population. Genes are considered to be derived characters because THEY ARE TRANSFER FROM GENERATION TO GENERATION FROM PARENTS TO THEIR OFFSPRING. Genes are derived from the DNA molecule of the parents and these are passed to their offspring during the process of cell division in reproduction.
They group together for warmth when it's cold and they also group together in case of threats.