Answer is: B. designing an experiment to test the plum pudding model.
Rutherford demonstrate that J.J Thompson's Plum Pudding model was not accurate, but before testing, he thought that plum pudding model is correct.
Rutherford theorized that atoms have their charge concentrated in a very small nucleus.
This was famous Rutherford's Gold Foil Experiment: he bombarded thin foil of gold with positive alpha particles (helium atom particles, consist of two protons and two neutrons).
Rutherford observed the deflection of alpha particles on the photographic film and notice that most of alpha particles passed straight through foil.
That is different from Plum Pudding model, because it shows that most of the atom is empty space.