To determine the active medium used in the first working laser ever constructed, we need to understand the concept of an active medium in a laser.

The active medium in a laser is the material that provides the energy necessary to create laser light. It is typically a solid, liquid, or gas that can be excited to produce a population inversion, which is the condition required for stimulated emission of photons.

Now, let's examine each option:

A. A diamond block: While diamond is a material that can be used as an active medium in certain types of lasers, it was not used in the first working laser ever constructed.

B. Carbon dioxide gas: Carbon dioxide gas was not used in the first working laser ever constructed.

C. Helium-neon gas: Helium-neon gas is commonly used in helium-neon lasers, but it was not used in the first working laser ever constructed.

D. A ruby rod: The correct answer is D. The first working laser ever constructed, known as the ruby laser, used a ruby rod as its active medium. The ruby rod was excited using a flashlamp, which created a population inversion and allowed for the production of laser light.

Therefore, the correct option is D. A ruby rod was the active medium used in the first working laser ever constructed.