The mass of an atom is contained primarily in its nucleus. The correct answer in regards to the given question is option "B". The nucleus is actually a dense compact region that is mostly made up of protons and neutrons and it takes the central position of an atom. Ernest Rutherford discovered the nucleus of an atom in the year 1911.The maximum weight of an atom is present in the nucleus and the remaining small amount of weight is taken up by the electrons surrounding the nucleus.