The correct answer is option 2: ecotone. An ecotone is the transition zone between two different plant communities or ecosystems. It is a boundary where the vegetation gradually changes from one type to another. Ecotones can be found in various natural habitats such as forests, wetlands, and grasslands. They are characterized by a mix of plant species from both adjacent communities and may have a unique set of environmental conditions and species that are adapted to the specific conditions found in the ecotone.

Option 1, ecological succession, refers to the process of gradual change in the composition of species within a community over time. It is not specific to the boundary between two plant communities.

Option 3, ecological niche, refers to the role and position of a species within its environment, including its interactions with other species. It is not specific to the transition zone between plant communities.

Option 4, climax, refers to the final or stable stage in ecological succession where a community reaches a state of equilibrium. It is not specific to the boundary between plant communities.

Overall, an ecotone is a unique and important area that supports a diverse range of species and plays a significant role in the functioning of ecosystems.