Answer
trans-2,3-diethyloxirane (or trans-3,4-epoxyhexane)
Work Step by Step
Since its a three-atom ring with one of them being O, it is an oxirane. There are two ethyl groups at carbon 2 and 3 respectively, so it 2,3-diethyl. And since they are trans to each other, it is trans-2,3-diethyloxirane. 3,4-epoxyhexane is also right because the oxygen is connected to the longest chain at carbon 3 and 4.
