Answer

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**Hint:**Mass number is actually the sum of total number of neutrons and protons present in the nucleus of an atom and It is approximately equal to the atomic weight of the atom, as mass of one neutron is almost equal to 1 amu and protons mass is also almost equal to 1 amu. Therefore it can also be said that atomic weight W is actually the sum of number of protons and number of neutrons.

**Complete Solution :**

Electron: It is the lightest stable subatomic particle known. It carries a negative charge of \[1.602176634\text{ }\times \text{ }{{10}^{-19}}~C\], which is considered the basic unit of electric charge. The rest mass of the electron is 0.000549amu, which is only \[\dfrac{1}{1836}\] of the mass of a proton. An electron is therefore considered nearly mass less in comparison with a proton or a neutron, and the electron mass is not included in calculating the mass number of an atom.

- Proton: proton is the stable subatomic particle that has a positive charge equal in magnitude to a unit of electron charge and a rest mass of 1.00727647amu.

- Neutron: Neutron is the neutral subatomic particle that is a constituent of every atomic nucleus except ordinary hydrogen. It has no electric charge and a rest mass equal to 1.0087 amu which is marginally greater than that of the proton but nearly 1,839 times greater than that of the electron.

Always remember that the number protons is actually the atomic number of the atom.

- Weight of the atom = number of protons + number of neutrons

W=N + Number of neutrons

Number of neutrons = W - N

**So, the correct answer is “Option B”.**

**Additional Information.**Neutrons and protons, commonly called nucleons, are bound together in the dense inner core of an atom, the nucleus, where they account for 99.9 percent of the atom’s mass.

**Note:**Quark is a fundamental constituent of matter and is defined as an elementary particle. These quarks combine to produce composite particles called hadrons, the most stable of which are neutrons and protons that are the components of atomic nuclei. Proton is made up of two up quarks and one down quark that give it a +1 unit charge. A neutron is made up 1 up quark and 2 down quark that make it a neutral particle.

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