Gout is known as “diseases of kings” because

The correct option is "of its association with rich food".

Historically, gout has been referred to as the "disease of kings" or "rich man's disease" because of its strong association with a wealthy lifestyle and diet.
In ancient and medieval times, only royalty and the very wealthy could afford a diet rich in meat, seafood, game, and alcohol (particularly wine and beer). These types of rich foods and drinks are highly concentrated in purines, which are natural chemical compounds found in cells.

When the body breaks down purines, it produces uric acid as a byproduct. If someone consumes an excessive amount of purine-rich foods, it can lead to a buildup of uric acid in the blood (hyperuricemia).
When uric acid levels become too high, it can form sharp, needle-like urate crystals in the joints, most commonly in the big toe, leading to the extremely painful inflammatory arthritis known as gout.

Since the common population historically lived on simpler, less purine-heavy diets and could not afford these rich foods, gout was predominantly diagnosed in wealthy individuals and monarchs, hence earning its nickname as the "disease of kings."