Correct Answer: Boon

Solution : The first option is the correct choice.

Boon: Something beneficial or advantageous; a blessing. This term is closely related to advantage in the sense that both refer to something favourable or advantageous.

The meanings of the other options are as follows:
Vital: It means essential or crucial to the existence or well-being of something.

Unfrock: It means to remove someone from a position of authority or privilege, often used in the context of removing a clergyman from the priesthood.

Vengeance: It denotes retribution or punishment in response to a perceived wrongdoing. This term is completely unrelated to advantage and carries a negative connotation.

Therefore, boon is the most appropriate synonym for advantage as it specifically denotes a favourable or beneficial factor.