Answer:
The smallest particle is the electron (option D).
Explanation:
The electron is a subatomic particle, which is part of an atom and has a negative electrical charge.
Electron is a small particle, if compared to protons -electrically positive particles- whose size is about 1800 times bigger. In fact, the electron is the smallest particle inside the atom.
According to this, the other options, atom, nucleus and proton are structures larger than an electron.
