This question asks you to evaluate whether the colonizing Western powers were good or bad for the areas which they colonized. To make your essay more streamlined, I would focus on one geopolitical region, such as Southeast Asia or Sub-Saharan Africa—two regions that were affected by colonialism well into the twentieth century.
If you would like to make the argument that the colonizers were good for the regions which they colonized, you can explore how the colonizers opened up natural resources in the area that are still major parts of the economies of the developing world today. You can also look at the limited infrastructure improvements such as dams and railroads that still benefit the countries in the developing world.
If you wish to argue that the colonizers were bad for the areas in which they colonized, you can look at the loss of native life and culture that came with colonization efforts. You can also look at the lack of development in these nations—the colonizers did not develop these regions so much as they exploited them for their own commercial benefit. This legacy of poverty continues today. You can also look at environmental degradation brought by the colonizers. You can also look at how the colonizers backed corrupt local officials who were friendly with white interests at the expense of helping native people in the area. This legacy of corruption continues throughout the developing world and holds the region back in terms of investment and development opportunities.
To formulate your five-paragraph essay, your first paragraph should introduce your argument so that the reader knows which side you are arguing--your argument can be summarized into the thesis statement. The next three paragraphs are where you will provide concrete examples for your argument. If you are arguing in favor of the colonizers, you will point out specific infrastructure developments in your region of choice. If you are arguing that the colonizers were a detriment, you can point out specific instances where native people were either exploited or killed. In your conclusion, restate your argument. This would also be a good place to point out how the actions of colonizers over one hundred years ago continue to affect the selected regions today.