Answer:
D. Asking clarifying questions
Step-by-step explanation:
According to the question, Frank politely interrupts Nia to ask if her information about coal mines apply to only the mines in the United States or all around the globe.
The discussion technique Frank is clearly using is asking clarifying questions. He asked the question to get clarity and avoid misconceptions because without his question, someone can wrongly (or rightly) assume that the information Nia gave is about all coal mines