Answer:
The best answer is "B"
When the reader may disagree with the bottom-line statement
Step-by-step explanation:
A Common ground is a point whereby opinions and interest is agreed upon by each party especially when they don't agree on a thing easily.
A bottom-line statement is more of a closing statement after which an agreement have been made. It is more like the conclusion of the whole statement.
Therefore the writer should establish a common ground before the bottom line statement when the reader may disagree with the bottom-line statement.