Answer: Option (b) is the correct answer.
Explanation:
As it is given that a dark grey ball has +1 charge. This means that it is a proton. And, a light grey ball has has 0 charge. This means it is a neutron.
Whereas a medium grey ball has -1 charge. This means that it is an electron.
As it is known that in a neutral atom the number of protons equal to the number of electrons.
Since, no overall charge is present on the given model of atom. Hence, the atom is neutral in nature.
Also, atomic number is the sum of total number of protons present in an atom. So, in the given atom total number of protons is 8, total number of neutrons is 9, and total number of electrons is also 8.
Thus, we can conclude that the atomic number of given atom is 8.