The correct answer is option 'C': Alpha particles.

Explanation:
1. Introduction:
- The question is related to the penetrating power of different types of radiation.
- The options given are: a) All have the same penetrating power, b) Beta particles, c) Alpha particles, d) Gamma rays.
- We need to determine which option has the least penetrating power.

2. Penetrating Power of Radiation:
- Penetrating power refers to the ability of radiation to pass through materials.
- Different types of radiation have different levels of penetrating power.
- The three common types of radiation are alpha particles, beta particles, and gamma rays.

3. Alpha Particles:
- Alpha particles are made up of two protons and two neutrons, which is identical to a helium nucleus.
- They have a positive charge and are relatively large and heavy.
- Due to their size and charge, alpha particles have a limited penetrating power.
- They can be stopped by a sheet of paper or a few centimeters of air.

4. Beta Particles:
- Beta particles are high-energy electrons or positrons.
- They have a negative charge and are much smaller and lighter compared to alpha particles.
- Beta particles have a higher penetrating power compared to alpha particles.
- They can pass through materials such as paper and thin aluminum, but can be stopped by a few millimeters of plastic or metal.

5. Gamma Rays:
- Gamma rays are electromagnetic radiation and have no charge or mass.
- They are highly energetic and have the highest penetrating power among the three types of radiation.
- Gamma rays can easily pass through most materials, including paper, aluminum, and even several centimeters of lead or concrete.

6. Conclusion:
- Based on the properties of the different types of radiation, it is clear that alpha particles have the least penetrating power.
- They can be stopped by a sheet of paper or a few centimeters of air.
- Beta particles have a higher penetrating power, and gamma rays have the highest penetrating power among the three.