The correct options are
B Trench warfare was a way to reduce vulnerability in the face of destructive weapons.
C In most cases involving trench warfare, the defender held the advantage.
D It was a trade-off between mobility and protection.
Trench warfare is a type of combat in which opposing troops fight from trenches facing each other. Trench warfare is resorted to when the superior firepower of the defense compels the opposing forces to dig in extensively so as to sacrifice their mobility in order to gain protection. Its origin can be traced to WWI, when the revolution in firepower was not matched by similar advances in mobility, resulting in a gruelling form of warfare in which the defender held the advantage. This form of combat was a remarkable feature of the western war front.