Consider the following statements.

A) Atoms of each element emit characteristics spectrum.

B) According to Bohr's postulate, an electron in a hydrogen atom, revolves in a certain stationary orbit.

C) The density of nuclear matter depends on the size of the nucleus.

D) A free neutron is stable but a free proton decay is possible.

E) Radioactivity is an indication of the instability of nuclei.

Choose the correct answer from the options given below.

Statement A is true, because atom of each element emits characteristics spectrum. Statement B is true,

 $\because$  According to Bohr’s postulates, $\mathrm{mvr}=\frac{\mathrm{nh}}{2\pi }$ and due to this reason, the electron resides into orbits of specific radius called stationary orbits.

Statement C is false, because density of nucleus is constant.

Statement D is false because a free neutron is unstable which decays into proton, electron and anti-neutrino.

Statement E is true because unstable nucleus shows radioactivity. So, we can say that radioactivity is the instability of nuclei.