Which part of the diagram represents the proton

C

Explanation:

An atom consists essentially of three types of particles:

- Protons: these are contained in the nucleus, and they are positively charged

- Neutrons: this are also contained in the nucleus, but they have no electric charge

- Electrons: they orbit around the nucleus, and they have negative charge

Based on this diagram, we see that the particle labeled by C is a proton, since it is in the nucleus and it is positively charged. The particle labeled by B is a neutron, while the particle labeled by A is an electron.