Final answer:
Alpha particles are the least penetrating among the given options. Neutrons, protons, and electrons are more penetrating.
Therefore correct option is D
Step-by-step explanation:
The most penetrating particle among the given options is alpha particles. Alpha particles are made up of two protons and two neutrons, which gives them a relatively large mass and charge compared to other particles. Due to their large size and higher electric charge, alpha particles have low penetration power and are easily stopped by a few centimeters of air or a sheet of paper. On the other hand, neutrons, protons, and electrons have smaller size, lower mass, and less charge compared to alpha particles, making them more penetrating.
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