Answer:

Introduction:
In an oligopoly market structure, a few large firms dominate the market. These firms often engage in strategic behavior to maximize their profits. One common form of strategic behavior is collusion, which involves firms reaching agreements to coordinate their actions and reduce competition. Among the given options, OPEC (Organization of the Petroleum Exporting Countries) is the best example of an agreement between oligopolists.

OPEC:
OPEC is an intergovernmental organization consisting of 13 oil-producing nations. It was established in 1960 with the goal of coordinating and unifying the petroleum policies of its member countries. OPEC member countries collectively control a significant portion of the world's oil reserves and production. This allows them to have a considerable influence on global oil prices.

Agreement and Coordination:
OPEC's primary objective is to stabilize oil markets and ensure a steady income for its member countries. To achieve this, OPEC members often engage in agreements to control oil production levels and manipulate prices. By coordinating their actions, OPEC members can collectively influence the global oil market. This agreement is an example of collusion between oligopolists.

Production Quotas:
One of the key strategies employed by OPEC is the implementation of production quotas. Member countries agree to limit their oil production to a certain level to maintain a balance between supply and demand. By restricting the supply of oil, OPEC aims to increase prices and maximize their revenues. This agreement allows OPEC members to collectively exert control over the global oil market.

Price Fixing:
Another form of agreement between OPEC members is price fixing. OPEC countries often discuss and agree upon target oil prices to ensure stability in the market. By collectively setting prices, OPEC members can avoid price wars and maintain profitability. This agreement helps oligopolists maintain their market power and control over the oil industry.

Conclusion:
Among the given options, OPEC is the best example of an agreement between oligopolists. OPEC's member countries engage in collusion to coordinate their actions, control oil production, and manipulate prices. Through production quotas and price fixing, OPEC members can collectively influence the global oil market and maximize their profits.