The practice of tying is used to: a. control the retail price of a collection of related products. b. package products to sell at a combined price closer to a buyer's total willingness to pay. c. encourage the enforcement of collusive agreements. d. enhance the enforcement of antitrust laws.

b. package products to sell at a combined price closer to a buyer's total willingness to pay.

Explanation:

Tying is a process under which a product is sold tied to some other product which ensures, that the buyer is not buying the product from somewhere else, and that the product is sold in combination with that of some other product at a price closest to the willingness of the buyer.

Thus, statement b) is most appropriate

that states that the combined product is sold at a price which is almost or nearby to the price which the buyer would be willing to pay.