Francium has the largest atomic radius. why might this be?

a. the highest number of protons for the least number of electrons and neutrons.
b. the highest number of energy levels and the least nuclear pull.
c. the lower ability to gain or lose electrons resulting in fewer ions forming.
d. the highest number of energy levels most nuclear pull.

explanation:

let the different atoms be x and y. each with four protons and same mass will be represented as follows.

mass number is number of protons + number of neutrons.

            and  

since, it is given that one has a negative charge while the other is neutral. therefore, there will be increase in number of protons and the atom will be represented as follows.

according to periodic table, mass number of beryllium is 9 and it has atomic number 4 so, the atoms will be and .  

the protons and neutrons will be located inside the nucleus whereas the electrons will be revolving around the nucleus.