Final answer:
The separation of Class A and Class X circuits in horizontal runs is determined by specific building codes and safety standards, which are generally designed to reduce electrical interference and increase safety.
Step-by-step explanation:
The question pertains to the separation of electrical circuits in horizontal runs, specifically Class A and Class X circuits. Although the specific code or regulation is not mentioned within the question, typically, building codes and safety standards dictate the specific distances required to separate different types of circuits. In general, these regulations are in place to minimize electrical interference, prevent potential faults, and enhance fire safety. Since the question has presented options ranging from 6 inches to 24 inches, if we are to assume the question is referencing a standard (such as the National Electric Code in the United States or another specific standard), the correct answer would likely be found within such documentation or taught in trade and vocational schools.