Which sentence is correctly punctuated?

The chosen sentence is punctuated correctly because it uses a colon appropriately to introduce an explanation or elaboration of what precedes it. In this case, the first part of the sentence sets up a statement about an agreement regarding an outcome, after which the colon indicates that what follows will provide specific details about that outcome. The phrase that follows the colon is a complete sentence that explains the outcome, making the use of the colon fitting.

In terms of punctuation rules, a colon is used before a list, an explanation, or an example and is especially appropriate when what follows directly elaborates on what was just mentioned. Here, the information about informed and uninformed participants clarifies and elaborates on the agreed-upon outcome.

The other options misapply punctuation rules. The use of a semicolon in one of the incorrect sentences suggests a close connection between two independent clauses but fails because "Informed participants perform better than uninformed participants" serves as an elaboration rather than a standalone, contrasting idea. This underscores the necessity of using a colon instead when providing such clarification. Overall, option C aligns with grammatical guidelines, making it the correct choice.