What is defined as the use of extremely chromatic harmony while still maintaining allegiance to a tonal center?

The correct answer is expanded tonality. Expanded tonality refers to the practice in music where a composer or musician uses chromatic harmony that goes beyond traditional tonal boundaries while still retaining a recognizable tonal center. This allows for a richer harmonic language, incorporating more complex chord structures and modulations, while still giving a sense of direction and resolution that a tonal center provides.

In contrast, modal scales focus on using modes that may not adhere to standard major or minor tonalities, which is different from the concept of expanded tonality. Polytonality involves the use of two or more different keys at the same time, creating a more dissonant texture, which contrasts with the idea of keeping a single tonal center. Jurassic tonality is not a commonly recognized term in music theory, making it an unlikely option in this context. Therefore, expanded tonality stands out as the most fitting definition for the use of chromatic harmony while maintaining allegiance to a tonal center.