B

Step 1: Understand the significance of Rutherford's α-particle experiment. This experiment was conducted in 1909 by Ernest Rutherford, who aimed to probe the structure of the atom by directing α-particles (helium nuclei) at a thin foil of gold.
Step 2: Observe the results of the experiment. Most α-particles passed through the foil with little or no deflection, but a small fraction were deflected at large angles, and some even bounced back. This indicated that a small, dense, positively charged center exists within the atom.
Step 3: Conclude that this dense center is the nucleus, which contains most of the atom's mass. The discovery of this nucleus was a pivotal moment in understanding atomic structure.
Step 4: Analyze the options:

• A: Proton - While protons are found within the nucleus, the experiment specifically highlighted the presence of the nucleus.
• B: Nucleus - Correct, as the experiment demonstrated the existence of a dense nucleus.
• C: Neutron - Neutrons were discovered later and were not observed in this experiment.
• D: Electrons - While electrons exist, the experiment focused on the nucleus.

Therefore, the correct answer is B.