Select the alternative which represents three out of the five alternative…

Concept

In a “three of five pieces reassemble the whole figure” question, first count how many regions the target figure is cut into, then note the shape TYPE of each region — how many sides it has, and whether it is convex or concave. A correct set of three pieces must reproduce that exact count and shape-type profile, not just a rough silhouette.

Applying it here

Look at how the equilateral triangle in the target image is divided, and at the five given pieces:

1. Two internal lines inside the equilateral triangle meet at a single interior point, cutting it into exactly three regions.

2. Tracing each region's boundary shows all three have four sides: the upper region is a concave, arrowhead-shaped quadrilateral, and the two lower regions are convex quadrilaterals.

3. Counting the sides of the five given pieces: piece (1) is a plain 3-sided triangle, piece (2) is also a plain 3-sided triangle, piece (3) is a 4-sided figure with a concave notch (an arrowhead shape), piece (4) is a 4-sided convex quadrilateral, and piece (5) is also a 4-sided convex quadrilateral.

4. Since every one of the three target regions is a quadrilateral, the matching set of three pieces must consist entirely of quadrilateral-type pieces — pieces (1) and (2), being plain triangles, cannot belong to this set no matter which other two pieces accompany them.

Cross-check

• Piece (3)'s concave notch lines up with the corner where the triangle's two internal lines cross, reproducing the arrowhead-shaped upper region.

• Pieces (4) and (5) are sized to exactly fill the two remaining convex regions on either side of that meeting point, with no gap or overlap.

• Together, pieces 3, 4 and 5 reconstruct the equilateral triangle exactly, confirming the set 345.