Final answer:
A buy-sell agreement principally establishes the price of the business, which is vital for the smooth transition of ownership when a co-owner leaves.
Step-by-step explanation:
The most important thing a buy-sell agreement establishes is D. the price of the business. A buy-sell agreement is a legally binding agreement between co-owners of a business that governs the situation if a co-owner dies, is forced to leave the business, or chooses to leave the business. It details how the share of the business will be reallocated, and most crucially, it sets the price at which the business share will be sold. This is vital to prevent disputes among the remaining owners and to ensure a smooth transition of ownership.