The salesman recommended that he keeps his receipt for at least a month.

The correct choice is "keeps," which is an example of a subjunctive mood usage in English. In this context, the phrase "that he keeps his receipt" should actually utilize the base form of the verb following "that," so "keep" should replace "keeps." The subjunctive mood is used to express wishes, recommendations, or suggestions. Here, the recommendation from the salesman necessitates using the base form "keep" to indicate that it is not a statement of fact, but rather a suggestion or recommendation.

The form "keeps" is in the present indicative, which is incorrect in this sentence because it implies certainty rather than a suggestion. Thus, the sentence should read: "The salesman recommended that he keep his receipt for at least a month."