What does it mean for a set to be "closed"?

The term "closed" in the context of sets generally refers to the **closure property**. This means that when you perform a specific operation (like addition, multiplication, etc.) on members of the set, the result is also a member of that same set.

Out of the options you provided, the most accurate interpretation of what it means for a set to be "closed" is:

**An operation performed on members of that set will result in a member of the same set.**

For example, the set of even integers is closed under addition because the sum of any two even integers is also an even integer. However, the set of even integers is not closed under division since dividing two even integers can yield a non-integer.

The other statements you provided are not correct in the context of set theory or the definition of closure:

- A "closed" set does not necessarily have to have a finite number of members; it can be finite or infinite.
- The statement regarding expressions belonging to a closed set is somewhat ambiguous and does not accurately describe closure in set theory.
- Members of a closed set can be added, subtracted, multiplied, or divided, but the critical point is that the result of these operations must remain within the set for it to be considered "closed."

So, the correct statement regarding what it means for a set to be "closed" is primarily linked to the closure property concerning operations defined on that set.