Economics: A group of competitors who work in unison to control the supply and price of their product is called a

cartel

monopoly

syndicate

trust.

cartel

Explanation:

A "cartel" is a group of competitors or market participants who are independent from each other. They work in unison by cooperating secretly in an unlawful way so they can control the supply and price of their products. In this way, they can dominate the market.

Such type of alliance with rivals have existed since the ancient times. It increased following World War I, but started declining after World War II.

So, this explains the answer.