Hey there!
Let's define our terms:
Voluntary muscle- A muscle that is controlled by the human will. For example, your arm is controlled by your will, and you move it when you want to. It doesn't normally automatically move by itself.
Involuntary muscle- A muscle that moves by itself without the will of the human. That means that it moves whenever it needs to, and doesn't require the human's will.
Using these definitions, we find that the muscles in the walls of the stomach and intestines are involuntary. This is because they move without conscious control.
Hope this helps!