Answer
The correct answer is (d) electrons at various energy levels.
Work Step by Step
An atom consists of fundamental particles, such as electrons, protons, and neutrons. Protons and neutrons are present in the nucleus and the electrons are in the state of revolving around the nucleus. The protons present in the nucleus carry a positive charge on them and the electrons revolving around the nucleus in their orbits carry a negative charge on them. Neutrons have no charge.
Among the three fundamental particles, electrons have very negligible mass of $9.11 \times10^{-31} kg$ .Protons have a mass of 1836 times that of the electron. Neutrons similarly have a mass of 1839 times the mass of the electron. Compared with protons and neutrons, the electrons have very negligible mass, and it gives no mass to the atom.
Protons in the nucleus, therefore, have a considerable mass, and it contributes to the weight of the atom. Therefore of the option (a) is an incorrect option.
Electrons revolve around the nucleus and they won't be present inside the nucleus. Hence, the option (b) is an incorrect option.
Neutrons in the nucleus also have a considerable mass, and it also contributes to the total weight of the nucleus. Therefore, the option (c) is also an incorrect option.
Thus, (d) electrons at various energy levels is the correct answer.
