Explanation:

- Option 1: Battle of the Somme was a significant and deadly battle in World War 1, but it was not the longest.

- Option 2: Battle of Mons was an early battle of World War 1, notable for being the first major action of the British Expeditionary Force, but it was not lengthy.

- Option 3: Battle of Verdun is correct. The Battle of Verdun was the longest battle of World War 1, lasting from February to December 1916.

- Option 4: The Battle of Jutland was a major naval battle but relatively short, taking place over two days.