Correct Answer:

Solution : The correct answer is alpha ray.

The rays which have shorter wavelength and lesser mass have more penetrating power. Alpha rays are positively charged helium atoms and they have considerable mass and charge because of which they don't have high penetrating power as compared to Beta rays, Gamma Rays and X-rays.

Among all the above options Gamma Rays have the highest penetrating power because they are neutrally charged rays and have shortest wavelength.