To get a line spectrum, the substances are excited in their
A) Solid state
B) Molecular state
C) Gaseous state
D) Atomic state

Hint: A line spectrum is formed when light is passed through a substance. The atom can absorb certain wavelengths of light from the incident light and excite the electrons.

Complete step by step answer:
When light is incident on a material, the electrons in the material can absorb certain wavelengths of light and be excited to higher levels. So, the emission and absorption spectra of the elements depend on the electronic structure of the atom. The process of obtaining the line spectrum is as follows:
A continuous spectrum of wavelengths of light is incident on the material. The material will absorb only certain wavelengths of light. These wavelengths of light will correspond to the energy transitions of electrons from lower to higher energy levels.
Now, when the external light is removed, the electrons in the higher excited states will fall back to lower energies and in the process, they will emit energy. The emitted energies will correspond to the difference of energies between the electron’s higher and ground states. Hence instead of a continuous spectrum, we will get specific lines corresponding to a certain wavelength which is known as a line spectrum.
Hence the excited electron states correspond to the atomic state of the material i.e. whether the electrons in the material are in an excited state or a ground state.

So, the correct choice is option (D).

Note: The line spectrum of a material is often used to detect which metal is present in a compound. A line spectrum is unique for every atom and molecule and acts like a fingerprint for identifying them. We can also say this way that the line spectrum is associated with the excited atomic state since it corresponds to excited electrons in the atom which can also be called an excited atomic state.