The phrase that should replace the vague pronoun "them" in context is "their brains," as is expressed in option D and explained below.
<h3>What is a vague pronoun?</h3>
We say a sentence has a vague pronoun when it is not clear to whom or what that pronoun refers. In sentence 4, the pronoun "them" is considered vague because we are not sure what it is about.
However, through context, we can tell "them" should be replaced by the phrase "their brains." The context reveals that "them" will be scanned by a brain imaging machine. What is scanned by such a machine? Brains.
With the information above in mind, we can select option D as the correct answer.
Learn more about vague pronouns here:
brainly.com/question/24465058
