How is good at electron configurations in chemistry?

Try to understand all the rules and laws like:

Aufbau Principle

Hund's rule

Pauli exclusion principle...

Then, you should understand the way in which you can fill the electrons in the orbitals!!

i'm assuming that the correct answer is indeed the first one/ "a"..

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.