What completes the circuit from the power source's positive terminal through the structure when the negative lead touches the structure?

The correct answer is the positive lead. In an electric circuit, a complete pathway is necessary for current to flow. The circuit is created when the positive terminal of the power source is connected through the positive lead to the structure and the negative lead connects to ground. When the negative lead touches the structure, it allows for the completion of the circuit as the current can return to the positive side through the positive lead, enabling the flow of electricity. This is fundamental to understanding how circuits work; they require both a source of voltage and a complete path back to that source for the current to function properly.