Final answer:
Correct option d. ground wire. Oleg should attach one probe of the multimeter to the ground wire when testing the circuit branch. This ensures safety by using the low-resistance path to earth provided by the ground wire.
Step-by-step explanation:
When Oleg is using a multimeter to test the circuit branch he's just installed, after turning off the current to the circuit at the service panel, he should first attach one probe to the ground wire in the end of the run. This is because the ground wire provides a low-resistance path directly to earth, which ensures safety and prevents electrical shock.
In electrical systems, the neutral wire is usually white and connected to earth at the voltage source and user location, keeping it at zero volts and safe to touch, even without insulation. However, when testing or troubleshooting a circuit with a multimeter, the ground wire is the appropriate choice for the initial connection of one probe.
This wire is typically green and is used to connect the metal case of an appliance to earth, ensuring that the case is also at zero volts. By attaching the multimeter probe to the ground wire, Oleg can safely and accurately test for proper grounding and circuit integrity.