Answer:
The presence of neutrons
Explanation:
Based on gold foil experiments, Rutherford proposed a atomic model known as Rutherford'd model or nuclear model or planetary model of the atom.
As per this model, most of the mass of the atom is concentrated in the center of the atom known as nucleus.
Electrons revolve around it same as planet revolve around sun.
Since, neutrons had not been discovered at the time of Rutherford'd model, therefore, as per the Rutherford'd model, nucleus is only made of protons.
Therefore, the correct option is option B
