For a narrative technique used in a biography to explain past circumstances and how they led to a person's current situation. This requires a method that delves into the past from the present narrativ

The question asks for a narrative technique used in a biography to explain past circumstances and how they led to a person's current situation. This requires a method that delves into the past from the present narrative.

• A. Outline: An outline is a structural plan for organizing information. While crucial for writing a biography, it doesn't explain past events within the narrative itself; it merely organizes the points to be covered.
• B. Flashbacks: A flashback is a literary device that interrupts the chronological sequence of a narrative to present an event that occurred earlier. In a biography, flashbacks are specifically used to provide background information, reveal past experiences, and show how previous events influenced the subject's present life and circumstances. This directly addresses the question's requirement.
• C. Engaging language: Engaging language is a stylistic element that makes a biography more interesting to read. It improves readability but is not a narrative tool for explaining past circumstances.
• D. Thesis statement: A thesis statement presents the main argument or central idea of a piece of writing. In a biography, it might summarize the person's life or its significance, but it doesn't narratively explain the progression of past events leading to the present.

Flashbacks are the most effective narrative tool among the choices for explaining past circumstances and their impact on a person's current situation within a biography.

The correct option is B.
$\boxed{\text{B. Flashbacks}}$