Answer
The charge conservation law would be violated.
Work Step by Step
The $\pi^-$ has charge $-1$
The down quark $d$ has charge $-\frac{1}{3}$
The up quark $u$ has charge $+\frac{2}{3}$
The net charge of the two quarks is $+\frac{1}{3}$
The charge conservation law would be violated.
