The correct answer is option 'C': A ruby rod.

- The active medium used in the first working laser ever constructed was a ruby rod.

- The first working laser was developed by Theodore Harold Maiman in 1960 at Hughes Research Laboratories.

- Maiman used a synthetic ruby crystal as the active medium.

- Ruby is a form of aluminum oxide (Al2O3) that contains traces of chromium ions (Cr3+).

- The chromium ions in the ruby crystal provide the necessary energy levels for laser action.

- The ruby rod was excited using an intense flashlamp, which generated a burst of light to pump energy into the crystal.

- The flashlamp caused the chromium ions in the ruby to absorb energy and reach a higher energy state.

- When the chromium ions return to their ground state, they release the absorbed energy in the form of photons.

- These emitted photons have a specific wavelength and are coherent, meaning they are all in phase with each other.

- The ruby rod is placed between two mirrors, creating an optical cavity.

- One mirror is fully reflective, while the other is partially reflective.

- This arrangement allows the photons to bounce back and forth between the mirrors, amplifying the light each time.

- As the photons pass through the ruby rod, they stimulate other chromium ions to emit more photons, resulting in a chain reaction of light emission.

- This process is known as stimulated emission and is the basis of laser operation.

- The partially reflective mirror allows a small portion of the light to escape, creating the laser beam.

- The first working laser using a ruby rod demonstrated the principle of stimulated emission and paved the way for the development of various types of lasers in different active media.