Answer :
It was trench warfare that was used to a great extent in World War I. This led to a significant elongation of the conflict, since targeting soldiers was much harder.
Thrench warfare was used extensively during World War I. The first option is correct.
Trench warfare consists on a type of land warfare using occupied fighting lines made up largely of military trenches, in which troops are well-protected from the enemy's small arms fire and are sheltered from artillery.
The most famous use of trench warfare is the Western Front in World War I, which was a global war originating in Europe that lasted from 28 July 1914 to 11 November 1918.
