In which bloody battle of the First World War did the British suffer a staggering 60,000 casualties on its first day?

The First World War, lasting from 1914 to 1918, was a conflict of unprecedented scale and brutality. One of its most infamous battles was the Battle of the Somme, fought in northern France. The battle was intended to relieve pressure on the French army at Verdun, but it became a symbol of the war's horrific trench warfare and immense loss of life. The first day of the Somme, July 1, 1916, remains the bloodiest day in British military history. British forces suffered approximately 60,000 casualties – killed, wounded, or missing – in a single day. This staggering number is why the Somme is the correct answer. The other options are battles from different periods of British history: Agincourt was a significant English victory during the Hundred Years' War in 1415; Waterloo marked the final defeat of Napoleon in 1815; and El Alamein was a pivotal battle in North Africa during the Second World War in 1942. While all significant, none match the sheer scale of British losses on the first day of the Somme.