MUTTER: INDISTINCT ::

A mutter is a term used to describe a sound that is barely audible. Indistinct also means the same (i.e, too low in volume to figure out the meaning). The terms given are synonyms. 

Define and easy are unrelated terms. 
Blunder means to commit a big mistake. Blunder and polished are unrelated terms. 
Expedite means to increase the pace of something. Expedite and completed are not related. 
Articulate is a term used to describe someone who communicates effectively. Well-spoken also means the same. Articulate and well-spoken are synonyms. Therefore, option C is the right answer.