Which system allows for the contents to be exposed to air upon opening?

Open systems are designed to allow for the exchange of air and possibly other materials with the surrounding environment. When an open system is opened, it permits the contents to be exposed to ambient air, which can facilitate specific processes such as mixing or application. This attribute makes open systems appropriate in certain scenarios where access to air is necessary for the intended use of the product.

In contrast, closed systems are meant to prevent any exchange with the external environment, maintaining a sterile or secure environment by containing the contents within a sealed barrier. Vacuum systems aim to create a negative pressure to prevent contamination while sealed systems focus on maintaining integrity and sterility by preventing air exposure and potential contamination.

Thus, the characteristic of allowing exposure to air is fundamental to the definition of open systems, making this the correct response in this context.