The building up of armed forces, weapons, and supplies reflects which cause of
World War 1?

militarism

Explanation:

Militarism is a belief or system where the military is exalted and its needs and considerations are given excessive importance or priority. Militarism was a powerful force in 19th and early 20th century Europe. While militarism alone did not start World War I, it fuelled a potent arms race and undermined the role of diplomacy as a means of resolving disputes.

sense of self-interest

i believe it would be a single seller : )