Two statements are given, one is labelled as Assertion (A) and the other as Reason (R):
Assertion (A) : London became the largest city in world by the start of twentieth century.
Reason (R): It had a population of 10 million by the start of twentieth century.

(A) is true , but (R) is false

The correct answer is Option 3: (A) is true, but (R) is false.

Assertion (A): London became the largest city in the world by the start of twentieth century. — Correct. London experienced rapid urban growth during the nineteenth century and emerged as the largest city in the world by the beginning of the twentieth century.

Reason (R): It had a population of 10 million by the start of twentieth century. — Incorrect. London had a population of around 6.5 million by the start of the twentieth century, not 10 million. The figure of 10 million is incorrect.

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 stillexpanding British empire.