Step 1: Formula for shear stress.
For thin-walled cylinders, the shear stress is inversely proportional to the thickness of the wall and directly proportional to the internal pressure and radius.
Step 2: Conclusion.
Thus, the correct expression for the maximum shear stress is \( \frac{pr}{2t} \).
Final Answer: \text{(B) \( \frac{pr}{2t} \)}
