Group the given figures into three classes using each figure only once.

Figure-classification (grouping) questions ask you to sort a set of figures into classes based on a shared STRUCTURAL relationship between their parts, such as one element being placed inside another, a figure being divided into equal parts by intersecting lines, or two similar elements of unequal size being joined together, rather than by their overall silhouette. Every figure must be used exactly once and must fall into exactly one class.

Examining the nine figures for these structural relationships:

• Figures 1, 3, and 9 each show one element placed inside a different element: a smaller box inside the cube's outline, a circle inside the octagon, and a smaller triangle inside the larger triangle.

• Figures 2, 5, and 6 are each cut into four parts by two mutually perpendicular lines.

• Figures 4, 7, and 8 each consist of two similar elements, unequal in size, joined to one another.

Assigning every figure to exactly one of these three classes gives the grouping 1, 3, 9 ; 2, 5, 6 ; 4, 7, 8, which matches the option listing this exact split.

As a check, no figure fits two classes at once: figure 6 is a symmetric four-part division rather than two attached elements, so it cannot sit with 4 and 7; and figure 9's smaller triangle sits inside the larger one -- a containment relationship -- rather than being two separate elements joined together, so it cannot sit with 4, 7, and 8 either. Each class ends up with exactly three figures, and all nine figures are used once.