Concept:An oasis is a fertile area in a desert where water is available at or near the surface.Explanation:Water in an oasis is derived from underground sources.It reaches the surface through natural springs or man-made wells.This underground water makes plant growth and human settlement possible in dry desert regions.Bajada, syncline, and sill are not sources of oasis water because they are landforms or geological structures, not water sources.Answer:B. Underground