One of the most important functions of botanical gardens is that:

One of the most important functions of botanical gardens is that they allow ex situ conservation of germplasm. Ex situ conservation refers to the conservation of plants outside of their natural habitat, and botanical gardens play a critical role in this process. So, the correct answer is option B. Notice that the other options are not correct. While botanical gardens may have tropical plants, provide a natural habitat for wildlife, and offer a beautiful area for recreation, these are not their primary functions.