In which time signature would you find a Compound Duple metre?

Compound Duple metre is characterized by a time signature that divides the beat into groups of three. In Compound Duple metre, there are typically two beats in each measure, with each beat further subdivided into three parts. Therefore, the correct choice is one that reflects this structure.

In the case of a 6/4 time signature, it consists of six beats in a measure, with each beat being a quarter note. This can be perceived as two groups of three beats, making it fit the definition of Compound Duple metre. The '6' signifies that there are six parts to the measure, and since each quarter note can be thought of as a triplet (or three eighth notes), this confirms its compound nature.

The other time signatures do not exhibit the same grouping. 3/4 is a simple triple metre with three beats per measure, 9/4 has a similar issue with being classified as a compound triple rather than duple, and 5/8 is a complex signature that does not align with the requirements of duple metre. Thus, the 6/4 time signature aptly represents Compound Duple metre.