Final answer:
Varying a theme by altering its melodic outline, rhythm, or harmony is called variation, a technique where the fundamental structure is recognizable while elements are modified for contrast, the final answer is D) variation.
Step-by-step explanation:
Varying a theme's melodic outline, rhythm, or harmony is known as variation. This is a musical technique where a composer or performer takes a theme and alters it in different ways to create a new, but related piece of music. When a theme is varied, its fundamental structure is recognizable, but elements such as its rhythm, harmony, or melodic contour are modified to provide contrast and interest. For instance, a simple melody might be played once in its basic form and then repeated with increased complexity or ornamentation. The other options in the multiple choice question suggest different musical concepts: Modulation refers to changing the key within a piece of music, improvisation is creating music spontaneously without premeditation, and ornamentation involves adding decorative notes to a melody to embellish it.
To decide the correct answer from the given options (A) modulation B) improvisation C) ornamentation D) variation), the key is to identify what best represents the adjustment of a melody's fundamental aspects, which in this case, is variation.