Alpha particles can be stopped by which of the following?

Alpha particles are relatively heavy and positively charged particles that are emitted during certain types of radioactive decay. Due to their mass and charge, alpha particles have limited penetration power compared to other forms of radiation such as beta particles or gamma rays.

The correct answer indicates that alpha particles can be stopped by just a few particles of air or a sheet of paper, which highlights their low penetration ability. Even a thin barrier can effectively absorb and stop them, as they interact strongly with matter due to their charge. This strong interaction leads to a rapid loss of energy, preventing them from passing through solid materials or even thin layers of gas.

In contrast, while options suggesting several meters of air, plastic sheets, and wood might imply greater barriers against radiation, alpha particles would not require such extensive obstacles to be stopped effectively. They would be absorbed much sooner than those scenarios suggest. Thus, the few particles within air or a thin sheet of paper are sufficient to completely halt the progress of alpha particles.