The major product of the following reaction when heated is:
Given a choice, hydrogen is abstracted instead of deuterium because the C−H bond is weaker.
For E2, we need carbon-hydrogen and the carbon-bromine bonds to be anti-periplanar.
They should be coplanar but the C−Br and C−H bonds should be on opposite sides.
After rotation, the antiperiplanar structure is in place for E2 (major product is):
For the minor product (only 17% yield), DBr is eliminated as: