The correct answer is: Alpha particles.
Alpha particles are the least penetrating type of radiation. They are positively charged helium nuclei that are emitted by radioactive atoms. Alpha particles have a very short range and can be stopped by a sheet of paper or even the outer layer of skin.
Beta particles are negatively charged electrons that are also emitted by radioactive atoms. Beta particles have a longer range than alpha particles and can be stopped by a thin piece of metal.
Gamma rays are high-energy photons that are emitted by radioactive atoms. Gamma rays have the longest range of all types of radiation and can only be stopped by thick layers of lead or concrete.
Here is a table that summarizes the properties of the three types of radiation:
| Type of radiation | Charge | Mass | Range |
| — | — | — | — |
| Alpha particles | +2 | 4 u | 0.0005 cm |
| Beta particles | -1 | 0.00055 u | 0.01 cm |
| Gamma rays | 0 | 0 | 10 cm |
I hope this helps!