Answer:
The correct answer would be D. brought Britain into the war
Explanation:
During this time, the British wanted no part of the First world war.
Germany couldn't breach the French Maginot line. So if going through, under, or over wasn't effective, why not go around it? This operation was called the Schlieffen Plan. This was pretty much attacking Belgium and going through Belgium to France.
Britain was allies with Belgium and didn't really like their allies being curve stomped by German troops.
(Simplified)