An atom is composed of protons, neutrons, and electrons. Protons have a positive charge, neutrons have no charge, and electrons have a negative charge. These particles are the building blocks of an atom and determine its properties. Protons and neutrons are located in the nucleus, while electrons orbit around the nucleus in energy levels. This arrangement of particles allows atoms to form bonds with other atoms, creating molecules and compounds. Therefore, the correct answer is protons, neutrons, and electrons.