What is the key characteristic of the Lydian mode?

The Lydian mode is characterized by its raised fourth scale degree, often notated as #4. This distinguishing feature sets it apart from the major scale, which contains a natural fourth. The presence of the sharp fourth contributes to the Lydian mode's unique, bright sound and creates a specific intervallic structure that can evoke various musical emotions and atmospheres.

In the context of the major scale, where the fourth degree is perfect, transforming it to a raised fourth adds an interesting tension and expands harmonic possibilities. This is particularly useful in composition and improvisation, allowing musicians to explore new melodic contours and harmonic ideas. The other options, while related to different musical modes and constructions, do not define the Lydian mode in the same way, making the raised fourth the key characteristic that identifies it.