A musical note is a sign used in musical notation. Option c is the correct choice.
A musical note is a symbol employed in musical notation to represent a specific pitch and duration. It is the fundamental building block of written music, conveying essential information to musicians regarding what pitch to play and for how long. Each note is positioned on the musical staff, indicating its pitch, and is accompanied by additional elements such as the note head, stem, and flags, which convey its duration.
Unlike an octave (option A) or a roman numeral (option B), a musical note directly represents a specific sound and its temporal duration in the musical language. It is distinct from a MIDI system (option D), which is a digital communication protocol for electronic musical instruments. Option c is the correct choice.