What can effectively block alpha particles?

Alpha particles, which are composed of two protons and two neutrons, have a relatively low mass and a positive charge. Due to their substantial mass and charge, they have low penetration ability and can be stopped by materials as light and thin as a sheet of paper.

The key factor is that alpha particles interact more strongly with matter due to their charge, leading to significant energy loss within just a few micrometers of material. Therefore, a sheet of paper is sufficient to obstruct their path completely, making it the correct answer in this context.

Other materials like aluminum foil, wood, and glass can also block alpha particles but are unnecessarily thicker and denser than what is required for effective shielding against them. Consequently, while they do function in blocking the particles, a basic sheet of paper is the simplest and most effective choice for this purpose.