Britain declared war on Germany as a consequence of Germany's attack on Belgium. Option C is correct.
The German invasion of Belgium was a military campaign which began on 4 August 1914.
Belgium's ports were close to the British coast and German control of Belgium would have been considered as a serious threat to Britain. In the end, Britain refused to ignore the events of 4 August 1914, when Germany attacked France through Belgium. Within hours, Britain declared war on Germany.