6. If 20 points divide a circle into 20 equal parts, then the number of regular polygons that can be formed with vertices only among these 20 points is ( ) .
(A) 4
(B) 8
(C) 12
(D) 24
6. (C).

Let the regular $k$-sided polygon satisfy the condition, then the 20 $-k$ points other than the $k$ vertices are evenly distributed on the minor arcs opposite to the sides of the regular $k$-sided polygon.
Thus, $\frac{20-k}{k}=\frac{20}{k}-1$ is an integer, so $k \mid 20$.
But $k \geqslant 3$,
$\therefore k=4$ or 5 or 10 or 20.
$\therefore$ The number of regular polygons sought is
$$
\frac{20}{4}+\frac{20}{5}+\frac{20}{10}+\frac{20}{20}=12(\uparrow)
$$