Answer : The correct option is, electron.
Explanation :
As we know that,
An atom is the smallest unit of a matter that consist of three subatomic particles which are electrons, protons and neutrons.
The protons and the neutrons are located inside the nucleus or the center of the nucleus where the mass of the an atom is concentrated.
The electrons are located around the nucleus.
The protons are positively charged, the electrons are negatively charged and the neutrons are neutral that means it has no charge.
According to the question, the electron subatomic particle found revolving around the nucleus.
Hence, the correct option is electron.