Lower wing, right
Step 1 / 5
d: Bottom inner layer: turn 90°, front face goes right
The front right edge has its upper wing and middle piece matching. The missing wing is low on the front left edge; its left color is the pair's front color.
d brings the wing over, R U R' stores the edge on top, d' undoes the slice.
Step 1 of 5: d. Bottom inner layer: turn 90°, front face goes right