Which type of radiation can penetrate paper but is stopped by a few millimeters of aluminum?

The correct choice is based on the properties of different types of radiation and their ability to penetrate materials. Beta radiation consists of high-energy, high-speed electrons or positrons. It has more penetration power than alpha radiation, which can be stopped by paper. However, beta particles can penetrate through paper and can only be stopped effectively by a few millimeters of aluminum or similar materials.

In contrast, gamma radiation and X-ray radiation are electromagnetic radiation that can penetrate materials much more deeply than beta particles and require denser materials, such as lead, for effective shielding. Alpha radiation, being relatively heavy and positively charged particles, has very limited penetration ability and can be easily blocked by a sheet of paper.

Therefore, the distinguishing characteristic of beta radiation is its ability to penetrate paper while being adequately stopped by a few millimeters of aluminum, making it the correct answer.