Fill in the blank with a suitable preposition.
This year there has been an increase _________ the number of books sold.
A. On
B. For
C. In
D. At

Hint:Prepositions are an integral part of speech. They are used to establish the relationship between the noun and the verb. The usage of Prepositions does alter with the type of subject (noun).

Complete answer:
1. In order to find the word that is the suitable preposition for this sentence, we will be checking each option one by one so that we can detect the correct option.
2.'On' is used to denote the place, specified time or direction. It cannot be used in the sentence given to us because it does not fit well grammatically.
3. The preposition 'for' can be used to refer to a space of time or to use of something. But it does not bring out the meaning of the sentence if used here.
4. 'In' is the correct answer to this question as it is used to indicate inclusion within space, a place, or limits, inclusion within or occurrence during a period of time. The given sentence indicates an occurrence with a given period of time.
5. 'At' refers to a specific location or to the time period when an event took place. This option is not correct as it cannot be used in this context.
6. Out of all these prepositions, the word 'in' suits the context of the given sentence in the question.

Hence the correct answer is option ‘C’

Note:If we see it more closely, we can find why the preposition ‘in’ is more apt than others as given in the options. An increment ‘in’ the numbers is being noticed. So, the process are carried inside the system or the numbered parameters.