What is the capital of Northern Ireland?

Belfast is the capital of Northern Ireland, making it the correct answer. It is not only the largest city in Northern Ireland but also an important cultural, political, and economic center. The city has a rich history, particularly during the period known as "The Troubles," and it continues to be a significant part of the United Kingdom. Located on the east coast of Ireland, Belfast is known for its maritime heritage and notable landmarks, such as the Titanic Belfast visitor attraction, which commemorates the city’s shipbuilding legacy.

Other locations listed in the question serve as capitals or major cities in their own regions: Cardiff is the capital of Wales, Glasgow is the largest city in Scotland, and Dublin is the capital of the Republic of Ireland. Each of these cities holds its own importance but does not pertain to Northern Ireland, thereby reinforcing that Belfast is the only correct choice regarding the capital of that region.