Final answer:
The electric field inside an infinite conducting cylindrical shell in electrostatic equilibrium is zero. Therefore, the correct answer is A. Zero.
Step-by-step explanation:
The electric field inside a conductive material in electrostatic equilibrium is zero. This is because any excess charge on a conductor resides on its surface, and if there were an electric field inside, the free charges within the conductor would move in response to that field until they canceled it out.
Applying this principle to the question, the electric field inside the infinite conducting cylindrical shell, regardless of its surface charge density or radius, is zero.
An electric field is a fundamental concept in physics that surrounds electrically charged particles and exerts a force on other charged particles within its influence. It's an invisible region in space where electrically charged objects experience a force due to their charges.
Thus, the correct answer to the question is A. Zero.