Explanation:

- In June 2017, NASA launched a mission focused on studying the interstellar medium. This is the matter that exists in the space between stars.

- Option 1: HESS - The High Energy Stereoscopic System (HESS) is an array of telescopes located in Namibia. It is designed for detecting gamma rays from cosmic sources.

- Option 2: FLESS - There is no known NASA mission or project by the name of FLESS related to studying interstellar medium.

- Option 3: BLESS - This also does not match any NASA project or mission related to the interstellar medium.

- Option 4: CHESS - CHESS (Colorado High-resolution Echelle Stellar Spectrograph) is an instrument NASA deployed in 2017 on a sounding rocket to study the interstellar medium and its properties.

Option 4: CHESS is indeed the mission launched by NASA. .