Currency
Flexible
Economy
Challenge
Security
It's a noun. The system and type of money that a particular country uses.
It's a noun. The state of feeling safe and being free from worry.
It's an adjective. That can be changed easily.
It's a noun. The operation of a country's money supply, comercial activities, and industry.
It's a noun. Something new and difficult that forces you to make a lot of effort.