A manager in a restaurant serves a 40-year-old adult a house drink special for $5 but charges a 21-year-old adult $9 for the same drink.

Price fixing involves an agreement between parties to set prices at a certain level, often eliminating competition. This situation does not involve collusion or an agreement to fix prices; it's an independent pricing strategy employed by the manager based on customer demographics, making this option incorrect.

Bait and switch is a deceptive marketing practice where a seller advertises a low price to attract customers but then persuades them to purchase a more expensive item. In this scenario, the manager is not using deceptive tactics or switching the product offered; instead, they are straightforwardly charging different prices, so this choice does not apply.

Predatory pricing refers to the strategy of setting prices extremely low in order to eliminate competition and gain market share. The charges of $5 and $9 do not reflect an attempt to undercut competitors but rather a differentiation in pricing based on customer characteristics. Thus, this choice is not relevant to the pricing tactic used here.