In the _________ procedure, participants are asked to say out loud what they are thinking while solving a problem.

The core of the question is about a procedure where individuals "say out loud what they are thinking while solving a problem." This is a method used to gain insight into cognitive processes.

• a. think-aloud protocol: This is a research method in cognitive psychology where participants are instructed to verbalize their thoughts, feelings, and reasoning processes as they complete a task. This directly matches the description.
• b. perceptual segregation: This refers to the process of distinguishing objects from their backgrounds or from other objects in a visual scene. It is related to perception, not verbalizing thoughts.
• c. anaphoric interference: This term relates to language comprehension, specifically interference that occurs when processing anaphoric references (e.g., pronouns) in text or speech. It is not a method for verbalizing problem-solving thoughts.
• d. interleaving: This is a learning strategy where different types of problems or topics are mixed during practice, rather than being studied in blocks. It is a learning technique, not a thought-verbalization procedure.

The "think-aloud protocol" is precisely the procedure described.

The final answer is $\boxed{\text{a. think-aloud protocol}}$.