The correct answer is agency's.
You said that you need the possessive form of the singular noun here. The singular noun is agency. You create the possessive form simply by adding an apostrophe (') and -s. So the possessive form of agency would be agency's.
On the other hand, the word agencies is plural - you would for the possessive form for plural nouns by adding the apostrophe only - agencies'.
Gatsby offers him a part in his " sideline business'' which in that case was bootlegging alcohol. Nicks response is that he is busy selling bonds with Walter Chase and that he does not need anything in response. This surprises Gatsby because he is used to having to pay for anything in return
Technically, freedom means "the ability's to act or change without constraint" but I believe its more than just that. When someone is free you can act, speak, think without anyone telling you what you can and cant do. Living with freedom is the right to be yourself.
Hope this helps :)