D is simply deteriorate. It's the isotope of H. So since D has a higher atomic weight, d must have a higher priority than H.
So this group has a higher priority than this group. Likewise, for the second carbon in a double bond, we have the following two groups. Once again, we have the H and we have the D.
So our D, the deteriorium, has a higher atomic weight, so it has a higher priority. And now we have an allochene where our two higher priority groups are on opposite sides of the double bond. And that means this must be an Eisomer.