What physical barrier can stop beta particles?

Beta particles are high-energy, high-speed electrons or positrons that can penetrate materials to a certain extent, but they are not as penetrating as gamma or X-rays. The correct choice, which is a layer of aluminum, effectively stops beta particles due to its atomic structure and density.

Aluminum serves as a sufficient barrier against beta radiation by absorbing the energy of the particles and preventing them from passing through. Its relative density and the fact that it is composed of a sufficient number of atomic layers allow it to disrupt the beta particles' paths, causing them to lose energy and eventually come to a stop.

The other options do not provide the same level of shielding against beta particles. A sheet of paper can stop low-energy beta particles, but it is not effective against higher-energy ones. A concrete wall can stop beta particles, but it is generally more suitable for gamma radiation and excessive in terms of practical applications when lighter materials like aluminum can suffice. A thin plastic sheet may not provide adequate stopping power for all beta particles, especially those with higher energy. Thus, a layer of aluminum stands out as the most effective and practical barrier for beta radiation among the given options.