Which of the following radiations is most easily stopped by air?
a. `alpha`-rays b.`gamma`-rays c. `beta`-rays d.X-rays

Explanation

To determine which type of radiation is most easily stopped by air, we need to analyze the penetrating power of each type of radiation mentioned: alpha rays, beta rays, gamma rays, and X-rays.

### Step-by-Step Solution:

1. **Understanding Radiation Types**:
- **Alpha Rays**: These are positively charged particles (helium nuclei) and have a relatively large mass. They have low penetrating power and can be stopped by a sheet of paper or even a few centimeters of air.
- **Beta Rays**: These are high-energy, high-speed electrons or positrons. They have a greater penetrating power than alpha rays but can still be stopped by materials like plastic or a few millimeters of aluminum.
- **Gamma Rays**: These are electromagnetic waves (photons) with very high energy and penetrating power. They can pass through most materials, including air, and require dense materials like lead or several centimeters of concrete to be effectively stopped.
...