What was the immediate cause of British entry into the First World War?

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The immediate cause of British entry into the First World War was the German invasion of neutral Belgium. Germany's violation of Belgian sovereignty and neutrality was seen as a direct threat to British security and a clear breach of international law. As a result, Britain declared war on Germany on August 4, 1914. The sinking of the Lusitania (A) occurred two years later in 1915 and was a contributing factor to the eventual entry of the United States into the war. The Austrian ultimatum to Serbia (C) was a trigger for the beginning of the war, but did not directly involve Britain. The Algeciras Conference (D) was a diplomatic meeting between European powers in 1906 and did not relate to the causes of the First World War.

What Was the Immediate Cause of British Entry into the First