Final answer:
J.J. Thomson concluded electrons must be in all atoms because atoms are neutral and electrons have a negative charge, meaning there must be a positive charge present to balance the electron charges.
Step-by-step explanation:
J.J. Thomson reasoned that electrons must be a part of atoms of all elements because electrons are negatively charged and atoms are neutral. This aligns with option A: Electrons are negatively charged and atoms are neutral. Thomson's discovery of the electron led to the understanding that since atoms are electrically neutral overall, they must contain positive charges to balance out the electrons' negative charges. His initial model for this arrangement was the plum pudding model, which theorized that electrons were embedded in a sphere of positive charge. However, his student, Ernest Rutherford, later disproved this model, leading to the current understanding that electrons orbit a central nucleus, which contains positively charged protons.