6. (3 points) Xiao Ming marks several points on the sides of a square, with exactly 3 on each side. The minimum number of points marked is
( ) .
A. 12
B. 10
C. 8
D. 6
【Analysis】To minimize the number of points marked on each edge, points must be marked at all four vertices of the square. Therefore, using the formula for the number of points around the perimeter of the square: number of points on each side $\times 4 - 4$, we can find the minimum number of points to be marked.

【Solution】Solution: $3 \times 4-4$,
$$
\begin{array}{l}
=12-4, \\
=8 \text { (points), }
\end{array}
$$

Answer: The minimum number of points to be marked is 8.
Therefore, the correct choice is: $C$.