Answer: The type of context clue in the sentence that helps explaining the meaning of the word "psychologist" is C. appositive.
Explanation: An appositive is a noun or a noun phrase that renames another noun that is found right before or after it. In that way, an appositive helps defining and providing additional information about a noun. In the sentence provided, "scientists who study the human mind" is an appositive since it has been separated from the rest of the elements in the sentence with dashes and it is helping to explain the meaning of the noun found right before it: "psychologist". Therefore, in this case, the appositive is working as a context clue.