Fill in the blank with the most appropriate word from the given optio…

The correct answer is Option (2) → lesser

Complete sentence: I found a lesser number of books in the library than what I found last time. The word "than" calls for a comparative, and "lesser" is the only comparative among the options that can stand before the noun "number": "I found a lesser number of books in the library than what I found last time".

Option	Meaning and use	Example
(1) less	Less goes before uncountable nouns (less water, less time). "A less number" is not used.	Add less sugar to my tea.
(2) lesser (answer)	Lesser is a comparative of "little" meaning smaller in amount, size or importance.	He agreed to accept a lesser amount than he had first asked for.
(3) few	Few is not a comparative, so it cannot go with "than". "A few number" is also wrong.	Few people came to the meeting.
(4) least	Least is a superlative. It compares more than two and cannot be used with "than".	He has the least experience of all the players.

Note: In careful modern English, "a smaller number of books" or just "fewer books" is preferred in this sentence, and "lesser" is not usually followed by "than". Among the given options, however, only "lesser" gives a comparative before "number", so it is the answer.