Explanation:

- Dell confirmed that two employees tested for the virus after a return trip.

- The context is about employees coming back to the U.S.

- Option 1: "of" does not fit here. It's used to show possession or belonging.

- Option 2: "from" is correct. It indicates the origin of the trip.

- Option 3: "to" is incorrect. It indicates direction towards a location, but doesn't match the context of returning.

- Option 4: "on" is used for dates or surfaces, not the return trip's origin.