The correct answer is Option A.
The conclusion plays an important role in any kind of writing. A well-written conclusion enhances the effectiveness of the writing.
As wrongly perceived, it is not the last line of the writing. Hence option B becomes the wrong answer.
Since option B is incorrect Option C too becomes incorrect- since it says both A and B.
The function of a conclusion in a report is defined as one that summarizes and highlights the main ideas of the reader and what the reader should be taking from the report. Thus Option A is the correct answer. (Therefore Option D - none of the above is incorrect.)