Correct answer is the second option: "The letter was <em>sent</em> by a disgruntled customer."
The first option refers to the decimal unit.
The third option refers to arranging for the transport of something.
The last option refers to a smell.
O The servant, the action took place on the stage.
The answer is C.
Tip:
Passive voice usually has the word "by" in it. Active does not.
hope this helps ya