Answer: Option (b) is the correct answer.
Explanation:
Atomic mass or mass number is defined as the sum of total number of both protons and neutrons present in an element.
For example, if an atom has mass number as 15 and its atomic number is 7.
Then it means that this atom contains 7 protons and the number of neutrons present in it will be calculated as follows.
Mass number = no. of protons + no. of neutrons
15 = 7 + no. of neutrons
no. of neutrons = 15 - 7
= 8
As mass of an atom is dependent on number of protons and neutrons and, these sub-atomic particles (protons and neutrons) are present inside the nucleus of an atom.
This means that mass of an atom is concentrated on the nucleus of an atom.
Thus, we can conclude that the mass of an atom is contained primarily by in its nucleus.
