In a voltaic cell, the salt bridge acts to:

provide a direct contact between the oxidation and reduction electrodes

provide an input of energy for the oxidation half-reaction.

provide constant overall balanced concentration of ions.

provide a surface for oxidation and reduction to occur.

The salt bridge acts to maintain a constant ion concentration for both of the half-cells. The anions are moving toward the oxidation side and the cations are moving toward the reduction side via the salt bridge.

Therefore the answer is C- provide constant overall balanced concentration of ions.

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.