6. Given that $n$ points are placed arbitrarily on the surface of a regular tetrahedron with side length $a$, there must be two points whose distance is no greater than $\frac{a}{2}$. The minimum value of $n$ is ( ).
A. 7
B. 8
C. 9
D. 10
6. A

Connecting the midpoints of two edges of a tetrahedron that share a common vertex divides the solid tetrahedron into 4 smaller tetrahedra and 1 central octahedron. In this octahedron, having 3 points ensures that there are at least two points with a distance no greater than $\frac{a}{2}$. Therefore, the answer is A.