The correct answer is (a) Mexico City.
According to the U.N. Population Report released in July, 2014, Mexico City is the most populous city in the world with a population of 21.8 million people. Mumbai is the second most populous city with a population of 20.7 million people, followed by Beijing with a population of 20.6 million people and São Paulo with a population of 19.8 million people.
Mexico City is the capital and most populous city of Mexico. It is located in the Valley of Mexico, a large valley in the center of Mexico that is surrounded by mountains. The city was founded in 1325 by the Aztecs and was originally called Tenochtitlan. After the Spanish conquest of Mexico, the city was renamed Mexico City and became the capital of the Spanish colony of New Spain. After Mexico gained independence from Spain in 1821, Mexico City became the capital of the new Mexican republic.
Today, Mexico City is a major economic and cultural center. It is home to many businesses, universities, and museums. The city is also a popular tourist destination.