National parks are large areas where the plant and animals are conserved in their own habitat. As they are conserved in their own habitat, they are best protected in the national parks.
Sanctuary - A wildlife reservation where animals or birds are conserved.
Botanical garden - An establishment where plants are grown for scientific study and display to the public.
Zoo - An establishment that maintains a collection of wild animals in a different habitat.
Thus, the correct answer is option D.