A chord made of tons only a half step or a whole step apart is known as a tone cluster .
Hence OPTION(D) is correct.
tone cluster :
A tone cluster is a musical chord comprising at least three adjacent tones in a scale . prototypically tone cluster are based on the chromatic scale and are separated by semitones . it is used in twentieth century music .
Polytonality :
The technique of using two or more tonal centres at the same time is called polytonality .
A polychord :
The combination of two traditional chords sounding together is known as a polychord . it consists of two or more chords ,one on the top of the other.
Bitonality :
use of two musical keys at the same time is called as bitonality.
Hence OPTION (D) IS correct.
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