The muscular tissue which functions throughout life continuously without fatigue is (b) cardiac muscle.
Explanation: Cardiac muscle, which is found in the heart, is designed to work tirelessly throughout a lifetime, constantly pumping blood without fatigue.
Why other options are incorrect:
(a) Skeletal muscle:
While skeletal muscles are important for movement, they do fatigue and require rest.
(c) Smooth muscle:
Smooth muscles are involuntary, meaning they contract without conscious control. They are found in organs like the stomach and bladder and can work continuously, but their function is different from the sustained, lifelong work of cardiac muscle.
(d) Voluntary muscle:
Voluntary muscles are those that are under conscious control, like skeletal muscles. They can fatigue and require rest.