(D) Cropland ecosystem
Cropland (crop-fields) is a human-made (artificial) ecosystem, whereas pond, grassland, and forest are natural ecosystems.
The passage explicitly states: "forests, ponds and lakes are natural ecosystems while gardens and crop-fields are human-made (artificial) ecosystems." Examiners expect you to correctly identify cropland as artificial. Grassland is natural, not mentioned as artificial, so do not confuse it with cropland.