Based on the description you provided—small, green, has chloroplasts, and all of its cells function in the same way—the organism is most likely a nonvascular plant (A).
Nonvascular plants, such as mosses, are typically small, have a green appearance due to chlorophyll in their chloroplasts, and rely on diffusion to transport nutrients, as they lack specialized vascular tissues found in vascular plants. While protists can also be green and have chloroplasts, they are usually unicellular, and this organism is described as multicellular. Therefore, option A is the best answer.