The instrument described is a clavichord.
Although not very popular nowadays, it was important between the 15th and the 19th century. In popular music, variations of it can be found in songs by artists such as Stevie Wonder and Bjork, but they mostly used the electric version of the instrument.
In opera, the libretto is the words or lyrics, as distinct from the music.
D.) analogous
these colors make up an analogous color scheme.