New species evolve when a group of individuals remains isolated from the rest of its species long enough to evolve different ... through mutation. fill in space

               Genotype ( genetic appearance )

Explanation:

                    When some individuals of a population  become isolated from the original population then  the two groups are also not able to reproduce together due to difference in genotype, So variations and mutations which causes change in genotype that occur in one group are not necessarily found in the other group which leads to speciation. The longer the groups are isolated, the more different they are they eventually become different species called speciation. When some individuals of a specie remain isolated from the rest of the members due to any type of barrier ( physical or geological) in between the individuals of both the original  species and currently isolated individuals so due to mutation the genotype of these individual changes and leads to speciation .

the correct answer is d