There are different mechanisms involved in whcih organisms are unable to reproduce.
1. The organisms G and H are separated geographically. These organisms are unable to mate because they cannot be in contact with each other.
2. The organisms I and J are not also able to reproduce offspring since their reproductive organs are incompatible with each other. The reason is probably caused by the shape of the reproductive organs, location, or the differences in size that inhibits the individuals from coupling because the sexual organs do not fit together. So, mating probably won't happen.
3. The organisms K and L are also unable to reproduce offspring because of the differences in their feeding mechanisms. This could be under behavioral isolation since the two organisms are different in some ways; hence they are incompatible with each other.