On the western front, the major military strategy that led to four long years of war was

a - the use of trench warfare

b - the use of tanks

c - the use of aerial reconnaissance

d - the use of calvary

my answer I say is? A.

a. the use of trench warfare

During World War I, the opposing sides dug into trenches to hold their ground. If they would try to mount an attack against the other side, venturing into "no man's land" generally meant getting mowed down by machine gun fire. In addition to trenches themselves, the soldiers would stack sand bags along the top of the trench to form a protective wall. The sand bag walls were effective against rifle fire, but not terribly effective against artillery fire from the opposing side. In addition to bullets and mortar fire, trench warfare also eventually involved the use of devastating chemical weapons also.

Trench warfare in World War I was miserable and gruesome. Check out All Quiet on the Western Front by Erich Maria Remarque (1929) for first-hand descriptions of the misery of the trench warfare.