Although the colon is often used to introduce block quotations, they can also be used as a formal way of introducing brief direct quotations.
A. used to introduce block quotations, they can also be used as a formal way of introducing
B. used to introduce block quotations, they can also be used to formally introduce
C. used as an introduction to block quotations, it has also been used for formally introducing
D. used to introduce block quotation, it can also be used as a formal way of introducing
E. using an introduction to block quotations, it can also be using for formally introducing
We need singular it for the correct answer. E uses wrong tense(is using implies that colon is using an introduction to block quotations right at this instance also the sentence wants to convey colon is used by someone not that the colon itself is using)
C uses has been and wrong idiom D does the job better