Question

Which sentence in this excerpt from Common Sense by Thomas Paine indicates that Great Britain protected the American colonies for mutual gain? A) That she hath engrossed us is true, and defended the continent at our expense as well as her own is admitted, and she would have defended Turkey from the same motive, viz., the sake of trade and dominion.B)Alas! we have been long led away by ancient prejudices and made large sacrifices to superstition C) We have boasted the protection of Great Britain, without considering, that her motive was interest not attachment D)Let Britain wave her pretensions to the continent, or the continent throw off the dependence, and we should be at peace with France and Spain were they at war with Britain. The miseries of Hanover last war, ought to warn us against connections.

Answer 1

The answer is letter C.
We have boasted the protection of Great Britain, without considering, that her motive was interest not attachment; that she did not protect us from our enemies on our account, but from her enemies on her own account, from those who had no quarrel with us on any other account, and who will always be our enemies on the same account. 
In this sentence, it is admitted that Great Britain protected the colonies, which would be for the gain of the colonies, but also that is did so for its own gain, so the gain was on both parts, "mutual" - I think that this is the correct answer.