The correct option is A Monoculture
Biodiversity refers to all the variety of life that can be found on earth. Monoculture is the cultivation of same crop in an area. Growing of a single crop in an area often leads to the loss of diversity of that area.
However, afforestation (planting of new trees) and biosphere reserves (areas that promotes the conservation of biodiversity) do not lead to the loss of biodiversity.