The correct option is B Nucleus of an atom
Thomson's atomic model is also known as plum pudding model.
According to this atomic model:
∙ An atom consists of a positively charged sphere with negative charges embedded on it.
∙ It failed to explain the concept of nucleus of an atom, which was later on discovered by Rutherford. Rutherford also discovered that the electrons revolve around the nucleus, which was not explained in the plum pudding model.