The correct answer is B.
The atom is made out of two parts that have the opposite charges, the positively charged nucleus (or the core) made out of protons and the negatively charged electrons that orbit the core.
The electrons, even though they posses the equal amount of electric charge as the protons are significantly smaller than the protons.
Therefore, the smallest particle in this line is the proton, followed by the proton, the nucleus, and in the end, buy the atom, the largest of them.
