Final answer:
The statement 'harmony is vertical' means that harmony involves notes that are sounded at the same time, forming chords. This is represented by option A. The concept of harmonics isn't directly related to this definition of harmony.
Step-by-step explanation:
The phrase "harmony is vertical" refers to the concept that harmony involves notes that are sounded at the same time, creating chords. This contrasts with melody, which is considered horizontal because it moves over time from one note to the next. In terms of the multiple choice options provided, the correct answer would be A. harmony involves notes that sound at the same time.
Harmonics, mentioned in the additional information, are related but a different concept. They are integer multiples of the fundamental frequency of a note and contribute to its timbre, or unique quality. However, this concept is not what is being asked about in the provided question.
It's important to note that harmony does not only include ascending pitches (B), nor is it about improvisation within a scale (C), and it certainly doesn't mean that harmony includes every note in an octave (D).