Fill in the blank with a suitable preposition:
Tokyo is the most crowded city _______ the world.
A. On
B. At
C. In
D. Upon

Hint:Prepositions are phrases that demonstrate associations with the other phrases in the sentence. Place prepositions are used to refer to a place, usually to indicate something or someone's location.

Complete answer:
 Prepositions are commonly short words, and they are usually put before nouns directly. It is a function word that generally combines with a noun term to form a phrase which usually expresses a modification or predication.

Prepositions signify direction, time, place, space and other abstract kinds of relationships. Let us look at examples for each. The prepositions are in italics for your convenience.
Direction: Look to the left and you will see our destination.
Time: We have been working since this morning.
Location: We saw a movie at the theater.
Space: The dog hid under the table.

Now, let us look at the given question. We already know, a preposition is a word that comes before a pronoun or a noun and creates a link between a clause or word elements. Option C is right because a part of something is suggested by it. As on, at, and upon indicate a surface, a particular location, and a surface that is above something, the other options are incorrect. These uses do not fit into the question given.

Hence the correct answer is option ‘C’

Note:If you get confused about which preposition will fit the best in the given blank, always refer to its relation with the noun. You can always look up in grammar books for better understanding.