Answer
See the detailed answer below.
Work Step by Step
$$\color{blue}{\bf [a]}$$
The atomic number of an element, denoted by $Z$, represents the number of protons in its nucleus. For example, hydrogen has an atomic number of 1, helium has 2, and lithium has 3.
If there were no other particles in the nucleus, the mass of helium would be expected to be twice the mass of hydrogen, and the mass of lithium would be three times the mass of hydrogen.
However, scientists discovered that the atomic mass of helium is actually four times the atomic mass of hydrogen, and the atomic mass of lithium is seven times the atomic mass of hydrogen. You can refer to the periodic table at the end of your textbook to find these atomic masses.
This means that there is something else within the nucleus that has no charge but has a big detected mass.
$$\color{blue}{\bf [b]}$$
The evidence was the isotopes of an element.
As an example, the hydrogen atoms, $Z = 1$, have been observed to possess integral masses of 1 u, 2 u, and 2 u.
Similarly, potassium atoms, $Z = 19$, exhibit integral masses of 39 u, 40 u, and 41 u.
Since the masses of these isotopes differ by approximately 1 u, they can assume that the mass of a neutron is approximately 1 u, which is identical to the mass of a proton.
