Based on the description you provided—a small green multicellular organism that has chloroplasts and whose cells function in the same way—it is most likely to be B) nonvascular plant.
Nonvascular plants, such as mosses, are small and contain chloroplasts for photosynthesis. While vascular plants also have chloroplasts, they are typically larger and have specialized tissues for water and nutrient transport. Bacteria are unicellular, and protists can vary widely in their characteristics but are often unicellular as well.