6. The lengths of the three altitudes of a triangle are $3, 4, 5$. When all three sides are the smallest integers, the length of the shortest side is ( ).
(A) 60
(B) 12
(C) 15
(D) 20
6. B.

Let the heights from $A$, $B$, and $C$ to the opposite sides $BC$, $CA$, and $AB$ of $\triangle ABC$ be $3$, $4$, and $5$, respectively. Then,
$$
2 S_{\triangle A I C}=3 BC=4 AC=5 AB \text{. }
$$

It is known that $AB$ is the shortest side.
Given $BC=\frac{5}{3} AB$, $AC=\frac{5}{4} AB$, $AB$ should be the least common multiple of $3$ and $4$, hence $AB=12$.