6. If convex hexagons are classified according to the number of axes of symmetry, then hexagons can be divided into ( ) categories.
(A) 3
(B) 4
(C) 5
(D) 7
6.C.

It is easy to find that hexagons with 0, 1, and 6 axes of symmetry exist. Now let's discuss the cases of 2, 3, 4, and 5. As shown in Figure 3(a), when lines $a$ and $b$ are both axes of symmetry of the hexagon, the hexagon must be a regular hexagon. Therefore, the number of axes of symmetry of a hexagon cannot be 4 or 5. Examples of hexagons with 2 and 3 axes of symmetry are shown in Figure 3(b) and (c), respectively.

Figure 3