Answer:
B. The development of nationalism among colonized peoples
Explanation:
A significant effect of Western imperialism in the twentieth century is "The development of nationalism among colonized people"
This is evident in the rise of agitation for self-rule or self-government whereby the countries in western imperialism are being practiced and for independence from colonial rule.
This is shown by the Indian movement for self-rule around the 1940s, and many African countries in the 1950s to 1970s
