B, a nucleus. Nuclei are membrane-bound organelles inside eukaryotic organisms. Eukaryotes are protists, fungi, plants, and animal cells. A bacteria is not a eukaryote, it is a prokaryote, so it does not have the membrane-bound structure of the nucleus.
Be careful, though, bacteria still have DNA. It is simply located in the center of the cell and is not encased in a membrane.