Harmony in art overall is achieved when the elements of an artwork come together in a unified way. Certain element can be repeated, yet they still look and feel like they are lending themselves to a whole. Harmony is definitely not monotony but also not chaos.
I know someone already answered, but since you only have four choices here I wanted to elaborate just in case! I’d go with unity, to have that element creates a general sense of wholeness or blended “harmony” in a work. Unity and harmony are sometimes used interchangeably in the art world so you can use whichever is listed as your option. hope this helps!! :)
