In demographic general knowledge, this question asks you to identify which city is considered the most populous city in the world when measured by metropolitan or urban agglomeration population.

Introduction / Context:
Understanding global population distribution is an important part of geography and economics. Many competitive exams ask about the largest countries and cities by population. When we talk about the most populous city in the world in terms of metropolitan or urban agglomeration population, one city consistently appears at the top of the list in recent decades. That city is Tokyo in Japan.

Given Data / Assumptions:
The question refers to the most populous city in the world.
The options are Tokyo, Paris, Peking, and London.
We interpret "most populous city" in the sense commonly used in GK books, which focus on urban agglomeration or metropolitan population.

Concept / Approach:
While many cities are large and famous, their populations differ significantly. Paris and London are iconic but no longer near the very top by population. Beijing, historically called Peking in older texts, has a large population, yet Tokyo has remained ahead in standard lists used for GK. Thus, answering this question correctly requires recalling updated rankings rather than relying only on fame or historical importance.

Step-by-Step Solution:
Step 1: Recall that various population lists rank Tokyo among the top global metropolitan areas by population. Step 2: Compare Tokyo with Paris, Peking, and London in terms of urban agglomeration size. Step 3: Recognise that Paris and London are important global cities, but their metropolitan populations are much smaller than that of Tokyo. Step 4: Note that Beijing (Peking) has grown rapidly, but standard GK sources and many exam keys still identify Tokyo as the most populous metropolitan city. Step 5: Choose Tokyo as the correct answer based on these widely used references.

Verification / Alternative check:
Demographic statistics compiled by international organisations and research groups consistently list the Greater Tokyo Area as having one of the largest urban populations in the world. While exact numbers change over time and some lists may place other cities close behind, exam oriented materials for general competitions still teach that Tokyo is the most populous city. Paris and London tend to occupy lower positions in these rankings.

Why Other Options Are Wrong:
Paris: A major European capital, but its metropolitan population is significantly lower than that of mega cities like Tokyo, Beijing, or Shanghai.
Peking: An older name for Beijing. Beijing is enormous, but commonly used GK lists still highlight Tokyo as the leader in metropolitan population over the recent decades that exam syllabi rely on.
London: Important historically and financially, yet its population remains behind Asian mega cities like Tokyo and Beijing.

Common Pitfalls:
Some candidates choose Peking because they are aware that Chinese cities have grown quickly and now have very large populations. Others may incorrectly choose London or Paris due to their global visibility in media. The key is to base your answer on standard demographic rankings used in textbooks and revision notes, where Tokyo is consistently identified as the most populous city in the world.

Final Answer:
The most populous city in the world is Tokyo.