When a sheet is bent, part of the strain it picks up is elastic and part is plastic. The plastic part is what stays behind permanently, but the elastic part behaves like a stretched spring that was never truly consumed. The moment the punch and die let go, that stored elastic strain relaxes back along the material's stiffness line, and the bend angle opens up slightly and the radius increases a little. This partial, unavoidable elastic rebound after the load is removed is exactly what shows up as springback, so the cause is elastic recovery, option (C).