Voluntary muscles are found in (b) limbs.
Explanation: Voluntary muscles are those that can be consciously controlled, and the muscles in limbs are the ones we use for movement, which is under our conscious control.
Why other options are incorrect:
(a) alimentary canal:
These muscles are smooth muscles, which are involuntary, meaning they are not under conscious control. They are responsible for the movement of food through the digestive system.
(c) iris of the eye:
The iris muscle is also smooth muscle and is involuntary, controlling the size of the pupil.
(d) bronchi of lungs:
These muscles are smooth muscles and are also involuntary, controlling the airflow in the lungs.