Charlie
Charlie comes from the International Flag signal - viz Code Charlie. When the the code pennant "Charlie" was hoisted two blocks at the yardarm it signified understanding the signal hoisted on another ship.
Our cousins in the New World naval forces loved to show off their maritime tradition by replying "Charlie" on voice for an affirmative. Likewise in the mess, when someone ripped in, they would shout Code Golf, meaning "Gas alert".
Somehow maritime traditions from the middle of the last century have crossed over the last fifty years into aviation.
BTW, It may actually be correct and proper to reply :Charlie" when radio propagation conditions are poor.