A can of Diet Coke.
Diet Coke was a brand of diet cola, produced by Coca-Cola, offered in the 20th century on Earth. It differed from the original drink by replacing the sugar with an artificial sweetener to reduce the beverage's calories.
In 1986, when Montgomery Scott and Leonard McCoy visited Plexicorp, Dr. Nichols had cans of Diet Coke in the fridge in his office. (Star Trek IV: The Voyage Home)