Answer
The angular momentum conservation law would be violated.
Work Step by Step
A proton's spin angular momentum is $\frac{1}{2}$
A neutron's spin angular momentum is $\frac{1}{2}$
A neutrino's spin angular momentum is $\frac{1}{2}$
If a proton decayed into a neutron and a neutrino, the angular momentum would not not conserved.
The angular momentum conservation law would be violated.
