Concept:The correct prepositional phrase is “at the instance of,” which means “at the request of” or “because of someone’s initiative.”
Explanation:The sentence says the meeting was called because of the chairman.
The word “instance” in this phrase means a request or an urging.
So the idiom “at the instance of” is used to show that someone caused an action to happen.
The chairman is the person whose initiative led to the meeting.
Therefore, the correct preposition is “at.”
The other options (“on,” “by,” and “for”) do not form a standard idiomatic expression with “instance.”
Answer:C. at