23. at the beginning of debussy's prélude à "l'après-midi d'un faune," the flute outlines
the interval of a
octave
unison
tritone
fifth

At the beginning of Debussy's Prélude à "L'Après-midi d'un faune," the flute outlines  the interval of a tritone.

Explanation:

Debussy's Prelude starts with the playing of single flute. There are 3 measures. In the piece of measure that is opening  the interval of tritone is outlined by the flute. This is done by playing scale in that descends  from C-sharp to G and then goes back again to C-sharp.

The fee of an ambiguity is imparted from this interval of tritone. The octave is partitioned into two halves. Here F-sharp, which is a tritone is above one C and a tritone below another C.  This similarity in the interval helps the music listeners to identify the pitch that is very important.  

c, d, e, f, g, a, b, and c.

sometimes western music will also use solfege. the names of these are do ra me fa sol la si (or ti) and do.

to indicate a note a half step higher or lower then the original note, you would say c # (for a half a step above c) or bb (for half a step below b)