Why did european nations form alliances in the early 1900s?
they were unsure they could protect themselves if attacked by larger nations.
they wanted more colonies in africa and asia.
they wanted to join with others who shared their culture and religion.
they wanted to form groups that made trade easier.

The main reason why European nations formed alliances in the early 1900s was that "They were unsure they could protect themselves if attacked by larger nations," since nationalism had caused nations to develop an "us vs. them mentality".

A)  They were unsure they could protect themselves if attacked by larger nations.

d is the best answer for this question.