The left half of a pattern is drawn on a dotted grid. Which right half completes it, so that the two halves match exactly when the sheet is folded along the dotted line?

The right half must be the mirror image of the left half in the fold line: every corner of the zigzag keeps its height and its distance from the fold. Only (d) does this; (a) is slid across without flipping, (b) is upside down and (c) has one corner at the wrong height.