A set has closure under an operation if performance of that operation on members of the set always produces a member of the same set; in this case we also say that the set is closed under the operation.
A closure property is a certain rule that holds if it is true for all elements of a set under the given operation and a closure property does not hold if there is at least one pair of elements that do not follow the closure property under the given operation.
Therefore, Emma is correct, because you can find a counterexample (7:2=3.5) that gives you a non-integer answer.
Answer: correct option is B
