6. The three external angle bisectors of $\triangle A B C$ intersect to form a $\triangle L M N$. Then $\triangle L M N$
(A) must be a right triangle;
(B) must be an obtuse triangle;
(C) must be an acute triangle;
(D) is not necessarily an acute triangle;
(E) must not be an acute triangle.
6. (C)

In the figure, $L, M, N$ are the intersections of the external angle bisectors of $\triangle ABC$, then
$$
\begin{array}{l}
\angle L A B=\frac{1}{2}(\angle B+\angle C) \\
\angle L B A=\frac{1}{2}(\angle A+\angle C) \\
\therefore \quad \angle L A B+\angle L B A \\
=\frac{1}{2}(\angle B+\angle C) \\
\quad+\frac{1}{2}(\angle A+\angle C) \\
=\frac{1}{2}(\angle A+\angle B+2 \angle C) \\
=90^{\circ}+\frac{1}{2} \angle C>90^{\circ} .
\end{array}
$$

Therefore, $\angle L<90^{\circ}$, which means it is an acute angle.
Similarly, $\angle M, \angle N$ are also acute angles. Thus, $\triangle L M N$ is an acute triangle.