The right answer is Chloroplast.
The chloroplast is a specific organelle of plant cells, probably derived from an endosymbiosis event of a cyanobacterium by a primitive eukaryotic cell. The chloroplast is indeed separated from the cytoplasm by a double membrane, two membranes separated by an intermembrane space.
Inside, the chloroplast consists of a stack of thylacoids, which bathe in the stroma (intrachloroplastic fluid).
