An atom is electrically neutral because the positive charges from its protons are balanced by the negative charges from its electrons. This occurs when the number of protons equals the number of elect

An atom is electrically neutral because the positive charges from its protons are balanced by the negative charges from its electrons. This occurs when the number of protons equals the number of electrons.

• A. electron is negatively charged. While true, this alone doesn't explain neutrality.
• B. neutron has no charge. While true, neutrons do not affect the overall electrical charge of an atom.
• C. number of electrons is equal to the number of protons. This is the correct reason. Protons carry a positive charge, and electrons carry an equal but opposite negative charge. When their numbers are equal, their charges cancel out, resulting in a neutral atom.
• D. proton and the neutron have an equal mass of one. This refers to atomic mass units and is not related to electrical neutrality.
• E. proton is positively charged. While true, this alone doesn't explain neutrality.

The correct option is C.

The final answer is $\boxed{\text{C}}$