In a ruby laser, the stimulated emission is due to the transition from
(A) metastable state to any lower state
(B) any higher state to lower state
(C) metastable state to ground state
(D) any higher state to ground state

Hint: In order to answer this question, we should first understand the meaning of a ruby laser. A ruby laser is a solid-state laser and it utilises the synthetic ruby crystal as its laser medium. It is among one of the few solid-state lasers that produce visible light and it emits a light which is red in colour.

Complete answer:
A ruby laser usually consists of a ruby rod which should be pumped with a very high energy, generally from a flashtube, in order to achieve a population inversion. The rod is placed between two mirrors which form an optical cavity which oscillate the light that is generated by the ruby's fluorescence thus resulting in stimulated emission.

Ruby is one of the very few solid state lasers which generate light in the visible range of the spectrum, lasing at \[694.3\]nm, in a red colour, with a narrow line width of \[0.53\] nm. This laser is a three-level laser. In this type of laser, the stimulated emission is because of the transition from a metastable state to a ground state.

Therefore, the correct answer is option C.

Note: It is important to note that ruby also absorbs some of the light at its lasing wavelength. In order to overcome this absorption, the entire length of the rod should be pumped, thus leaving no shaded areas near the mountings. The active part of the ruby is the called the dopant and it consists of chromium ions suspended in a synthetic sapphire crystal.