Final answer:
The best generalization about early WWI warfare is that few territorial gains were made and casualties were high due to trench warfare and deadly new technologies.
Step-by-step explanation:
The statement that makes the best generalization about warfare in the early years of World War I is C. Few territorial gains were made, and casualties were high. This period of the war was marked by prolonged trench warfare, with major battles like Verdun and the Somme resulting in massive casualties and little to no significant territorial advancements. The technology of the time, including machine guns and poison gas, created a deadlock where defensive positions were far more advantageous than offensive attacks, leading to high casualty rates without commensurate gains in territory.