Answer : The correct option is, gamma
Explanation :
Penetration power : It is the movement of rays or the particles through an object. We know that the more the energy of a particle, the more will be the penetrating power.
The increasing order of the penetration power of alpha rays, beta rays, and gamma rays will be,
Alpha rays < Beta rays < Gamma rays
The penetrating power of the alpha, beta and gamma rays varies greatly.
(1) The alpha particles are restricted by the paper.
(2) The beta particles can pass through the paper but it can not pass through the aluminum foil.
(3) The blocking of gamma rays is very difficult. It can be blocked only by concrete, lead, or other heavy shielding.
Hence, the radiation can only be stopped by thick layers of dense metal is, gamma.
