The answer is Photosphere Apex.
Answer : Option 4) Region of the most probable electron location.
Explanation : As per the electron cloud model of the atom, an orbital is a region where the probability of finding an electron is highest. According to this model which was used to identify the probable location of the electrons when they go around the nucleus of an atom.
This electron cloud model was different from the older Bohr atomic model by Niels Bohr.
Mass number is equal to the number of protons plus the number of neutrons in a nucleus of an atom