Question

harmony means :
the act of changing something slightly from the norm
the structure of music with respect to composition of chords
a system for transmitting messages in brevity or secrecy
an indication of the existence of some person or thing

Explanation:

Harmony in music refers to the combination and structure of chords. Changing something slightly from the norm is mutation - like concept. A system for transmitting messages is a code - related concept. An indication of existence is a sign - related concept.

Answer:

B. the structure of music with respect to composition of chords