Identify the correct punctuation revision: "The participants were introduced to each of the trainers, but they were not allowed to choose their own trainer."

The original sentence, "The participants were introduced to each of the trainers, but they were not allowed to choose their own trainer," is correctly punctuated with a coordinating conjunction ("but") preceded by a comma. This kind of punctuation is necessary when joining two independent clauses, as it aids in providing clarity and indicating the relationship between the two ideas.

In this case, the first independent clause discusses the introduction of participants to trainers, while the second clause presents a contrasting idea regarding their inability to choose their preferred trainer. The comma before "but" effectively signals this contrast, making the sentence easier to read and understand.

Other options either remove essential punctuation or incorrectly separate clauses in a way that disrupts the flow or comprehension of the sentence. For example, leaving out the comma creates confusion by merging two distinct ideas without the necessary pause, and using a semicolon improperly connects the independent clauses in a manner that is not typical for the conjunction being used.