Thought tracking is best described as:

Thought tracking is a technique used in drama that allows a character's internal thoughts and feelings to be expressed directly to the audience. By speaking a character's thoughts aloud, actors can provide insight into their motivations, dilemmas, and emotional states, effectively bridging the gap between the audience and the character’s inner world. This technique enhances the audience's understanding of the character's perspective and creates a more engaging theatrical experience.

The other options, while related to character development and performance, do not capture the essence of thought tracking. Exploring character history involves delving into past events that shape a character, while writing letters from a character's perspective focuses on developing character voice and backstory through written format. Performing improvisational scenes emphasizes spontaneity and actor collaboration rather than the individual articulation of a character's thoughts. Thus, speaking a character’s thoughts aloud is the definitive aspect of thought tracking.