I believe it's B. Cytoskeleton and Cytoplasm.
The mass of an atom is contained primarily in its NUCLEUS.
An atom is made up of three sub particles, which are proton, neutrons and electrons. The proton and the neutrons are located in the nucleus of the atom while the electron are located outside the nucleus where they move about in continuous motion around the nucleus. The proton and the neutrons, which are located in the nucleus have the same relative mass of 1, which has been estimated to be about 2,000 times bigger than that of the electron, which has a relative mass of 0.0005. This shows that, the mass of an atom is located primarily in its nucleus. <span />
