The relationship in the analogy Current : Circuit :: Earth : ? is:

Thing that Travels : The Path upon which it Travels

• First Pair (Current : Circuit): An electric current travels along a defined path called a circuit.
• Second Pair (Earth : ?): Following the same logic, the planet Earth travels along a defined path as it revolves around the Sun. This path is called its orbit.

Conclusion: "Orbit" is the correct word to complete the analogy because it represents the path on which the Earth travels, just as a "circuit" is the path on which a "current" travels.

Final Answer: The correct option is B. Orbit.