Select the term that describes the process of creating a musical line that is derived from a previously established line.

The term that accurately describes the process of creating a musical line that is derived from a previously established line is imitative counterpoint. This technique involves taking a melody or musical idea and having another voice or part mimic or echo it, often at different pitches or in different rhythms. This practice is fundamental in counterpoint, where the interaction between melodic lines creates a rich texture.

Imitative counterpoint not only establishes a thematic relationship between the lines but also explores the development of ideas in a conversational manner, where one part states a theme and subsequent parts respond with variations of that theme. In this way, the original melody influences the new lines and leads to an intricate interplay of musical voices.

The other options relate to ways of developing themes or musical ideas but do not specifically refer to the imitation aspect inherent in imitative counterpoint. Theme development focuses on elaborating a single theme, variation technique refers to altering a theme while maintaining its identity, and motivic expansion involves taking small motives and developing them through various means, none capturing the essence of imitation as effectively as imitative counterpoint does.