Any pipe installed underground must have at least ______ inches of clearance between the outside of the pipe and the extremity of any other underground structure (pipeline).

According to 49 CFR 195.250, "Clearance between pipelines and other underground structures," any pipe installed underground must have at least 12 inches of clearance between the outside of the pipe and the extremity of any other underground structure (pipeline).

The final answer is $\text{b. 12}$