Which phrase best defines “quotation”?
a source excerpt in your own words
a brief statement of the main ideas of a text
a sentence written on an index card
a word-for-word excerpt from a text

The best answer is "a word-for-word excerpt from a text." You use a quotation when you cite a source or a person directly. It is used with quotation marks and must be properly cited in any academic paper.
"A brief statement of the main ideas of a text" is a summary, and "a source excerpt in your own words" is a paraphrase. 
"A sentence written on an index card" can be anything - depends on what the sentence is!
You'll notice I copied the words you wrote in quotation marks. Those are quotations, because you said them, and I didn't. Hopefully this helps to illustrate this concept. :)