Which of the following is NOT a requirement for the Robertson-Patman Act to be enforced?

The correct answer, which identifies what is NOT a requirement for the enforcement of the Robertson-Patman Act, emphasizes that the law is primarily concerned with price discrimination practices rather than the nature of the commodities being sold.

The underlying purpose of the Robertson-Patman Act is to ensure fair pricing among competing purchasers and to protect competition by preventing sellers from charging different prices to different buyers for the same product. Thus, while the existence of at least two sales, a difference in price, and two or more purchasers are indeed fundamental aspects required for establishing a violation of the act, the sales having to involve different commodities is not a stipulation. This means that the act can enforce against discriminatory pricing even if the sales involve the same type of commodity; it's the pricing strategy that is critical.

The incorrect options revolve around the act's focus on transactional conditions necessary for proving an unfair pricing strategy—namely, that there must be multiple sales made to at least two purchasers and that these sales involve differing prices.