Answer:
C. When the reader may disagree with the bottom-line statement
Step-by-step explanation:
A common ground can be regarded as an area of shared interests which is been held number of people or groups. It is a point at which opinions and interest is been agreed upon by parties. A bottom-line statement can be regarded as a likely closing statement made after an agreement has been reached, it's just like a conclusion after the whole statement. Hence, it is necessary for the writer to establish a common ground first before he/she will establish bottom line statement "when the reader may disagree with the bottom-line statement''