(A) ** longest & costliest
(B) longest & most violent
(C) costliest & coldest
(D) most difficult & deadliest
EXPLANATIONS BELOW
Concept note-1: -The Battle of Verdun, 21 February-15 December 1916, became the longest battle in modern history. It was originally planned by the German Chief of General Staff, Erich von Falkenhayn to secure victory for Germany on the Western Front .
Concept note-2: -A deadly combination of strategy and circumstances ensured Germany and France fought on for nearly 10 months, even when the largest swathe of territory gained amounted to a mere five miles. Pride, politics and tactics would all play a role in prolonging one of the deadliest conflicts of the Great War.
Concept note-3: -In a war known for its brutality, the Battle of Verdun, (February 21–December 18, 1916), was among the longest and most bloodiest conflicts of World War I. In the battle that slogged on for 10 months, the French held off a major German offensive.
Concept note-4: -The battle lasted for 302 days, the longest and one of the most costly in human history.
Concept note-5: -The Battle of Verdun (21 February-18 December 1916) was the longest battle of the First World War. It was also one of the costliest. It began in February 1916 with a German attack on the fortified French town of Verdun, where bitter fighting would continue for most of the year.