Choose the meaning of the underlined idiomatic phrase: The stories of senior citizens in distressed economic conditions because of defaulted student loans are merely the canary in the coal mine. [Washington Post]

Good tidings for future

Having advantageous position

Harbinger of serious problem

Not much coal remaining in the mine

The correct meaning is: "Harbinger of serious problem."

The phrase "canary in the coal mine" refers to an early warning or indicator of a larger issue.

In this context, it means that the stories of seniors with economic distress due to student loans signal a broader, more serious problem.