Which city became the largest city in the world by the start of the 20th Century?

London

The correct answer is option 2: London

NCERT:"In 1800, well under 20 per cent of the population lived in towns or cities of more than 10,000 inhabitants. By 1900 this proportion had become 74 per cent. The capital city, London, was home to about 1.1 million people in 1800; it increased in size to a population of over 7 million by the start of the twentieth century. London was then by far the largest city ever seen in the world, a vast manufacturing, commercial and financial centre at the heart of a still-expanding British empire. (Giddens 2001: 572)"