What characterizes a Compound Duple time signature?

A compound duple time signature is characterized by having two beats per measure, with each beat being divided into three equal parts. This means that each beat consists of a dotted note value, which gives the music a lilting or flowing quality.

In this context, 6/4 fits this definition perfectly: it has six beats in total, and it is typically grouped into two measures of three beats each, often notated in a way that emphasizes the compound nature. The overall feel of 6/4 allows it to be counted as two beats of dotted quarter notes, and this is why it's classified as compound duple.

On the other hand, the other options do not represent compound duple time signatures. For example, 9/4 signifies a compound triple time because it has three beats in each measure, while 12/4 represents a compound quadruple time with four beats per measure. 3/4 is considered simple triple time as it consists of three beats, each divided into two parts. Understanding these distinctions is important for identifying various time signatures in music.