The example that best illustrates disputes over political boundaries being influenced by informal boundaries is:
A country annexes a region that contains a religious group that is a majority in the country.
This situation highlights how informal boundaries, such as cultural, ethnic, or religious affiliations, can affect political decisions regarding formal political boundaries. The presence of a majority religious group in a region may motivate a country to annex that region to consolidate power or identity based on shared beliefs or affiliation, thus blurring the lines between political and informal boundaries.