The interval between any note and its nearest neighbor with the same letter name is called
a. an octave
b. a unison
c. a third
d. a fifth

(A) an octave

Explanation:

an octave is a series of eight notes occupying the interval between (and including) two notes, one having twice or half the frequency of vibration of the other.

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)