Which of the following defines durational rhythm?

The correct answer highlights durational rhythm's essential characteristics, which focus on how long notes are held, their relative lengths, and the emphasis placed on certain beats or notes within a musical piece. Durational rhythm goes beyond mere beat patterns, incorporating the aspect of time and the relationships between notes in terms of duration. This is crucial for creating distinct rhythmic patterns that contribute to the overall feel and groove of the music.

The other options do cover different aspects of music but do not directly address durational rhythm. Patterns of beats, while related, do not encapsulate the focus on duration and emphasis that defines durational rhythm. Melodic contours pertain more to the pitch and shape of melodies rather than their rhythmic properties. Structural forms refer to the overall organization and layout of a piece of music, which involves how different sections are arranged rather than the specific timing and emphasis of individual notes.