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

The bottom half must be the mirror image of the top half in the horizontal fold line: each corner keeps its left-right position and its distance from the fold. (a) is also flipped left to right, (c) is slid down without flipping, and (d) has one corner out of place, so the answer is (b).