The correct answer is C. Conservation of a species outside its natural habitat.
Ex-situ conservation is the conservation of plant or animal species outside of their natural habitats. This can be done in a variety of ways, such as in zoos, botanical gardens, or wildlife rehabilitation centers. Ex-situ conservation is often used to protect species that are endangered or threatened in the wild. It can also be used to breed endangered species in captivity, which can then be released back into the wild.
Option A is incorrect because it describes in-situ conservation, which is the conservation of a species in its natural habitat. Option B is incorrect because it is too broad. Conservation can refer to the protection of any type of ecosystem, not just forests. Option D is incorrect because it is too narrow. Conservation can refer to the protection of any type of organism, not just plants.