The correct answer is C. Alpha rays.
Alpha rays are the least penetrating type of radiation. They are made up of two protons and two neutrons, which are bound together to form a helium nucleus. Alpha particles are positively charged and have a mass of about 4 atomic mass units (amu). This makes them very easy to stop, and they can be stopped by a sheet of paper or even a few layers of clothing.
Beta rays are made up of electrons, which are negatively charged particles with a mass of about 0.5 amu. Beta particles are more penetrating than alpha particles, but they can still be stopped by a thin sheet of metal.
Gamma rays are high-energy photons, which are similar to light but have much higher energy. Gamma rays are the most penetrating type of radiation, and they can only be stopped by thick layers of lead or concrete.
In conclusion, alpha rays are the least penetrating type of radiation. They are made up of two protons and two neutrons, which are bound together to form a helium nucleus. Alpha particles are positively charged and have a mass of about 4 atomic mass units (amu). This makes them very easy to stop, and they can be stopped by a sheet of paper or even a few layers of clothing.