Of the events listed here, the best answer is that Great Britain went to war because Germany invaded Belgium. Not only did the British have a treaty in which they had promised to protect Belgium’s neutrality, but they also felt that it would be dangerous to have the Germans in control of Belgian ports so near to England.
The other options that are given here are not correct answers. First, Option C is completely incorrect because Germany did not invade Austria. Instead, Germany and Austria were allies in this war so Germany would not have invaded that country. Option B is somewhat better because Germany and Russia were at war with one another. However, Germany did not invade Russia at the beginning of the war. By the time Germany did invade, Britain had been in the war for a long time, meaning that this invasion had nothing to do with Britain entering the war.
Instead, Britain entered the war because Germany invaded Belgium as a way of getting their armies to France. Britain had a treaty that guaranteed Belgium’s territorial integrity. In addition, Britain did not want Germany to have control of Belgium’s ports since those were extremely near to England. For these reasons, Britain went to war with Germany when Germany invaded Belgium.