The correct answer is C. A ruby rod.
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The first working laser was built in 1960 by Theodore Maiman at Hughes Research Laboratories. The active medium in Maiman’s laser was a ruby rod.
A ruby rod is a cylindrical rod of synthetic ruby, a mineral composed of aluminum oxide with trace chromium impurities. The chromium impurities give ruby its red color. When a ruby rod is placed in a high-energy environment, such as a flash lamp, the chromium atoms are excited to a higher energy state. When these excited atoms decay back to their ground state, they emit light of a specific wavelength. This light is amplified by the other chromium atoms in the rod, and is emitted as a laser beam.
The other options are incorrect. A diamond block is not a suitable material for a laser because it does not have the necessary optical properties. Helium-neon gas and carbon dioxide gas are both used in lasers, but they were not the active medium in the first working laser.