Answer:
Closed system
Explanation:
In order to do this, let's remember the concepts of an open, closed and isolated system.
Open system: Any matter and heat can enter or leave the system.
Closed system: any matter cannot enter or leave the system, however heat can enter or leave the system.
Isolated system: neither matter nor heat can enter or leave the system.
Now, an open system cannot be, cause the flask is sealed at the top with a balloon, so matter cannot escape the system.
Now, in general terms, an erlenmeyer flask is not insulated to prevent that any heat or energy can be transfered, hence, the system, even with the balloon to prevent any escape of the gaseous product, it can still transfer heat or energy to the medium. This can be verified by touching the balloon or flask, therefore, is a closed system.