Final answer:
A multimeter is the correct tool to test for opens and grounds in the field coils of a starter motor. It can measure resistance and ensure that no voltage is present before testing.
Step-by-step explanation:
When testing the field coils of a starter motor for opens and grounds, the most appropriate tool to use is a multimeter. A multimeter can measure resistance, continuity, voltage, and current which are vital in checking the integrity of the field coils. To perform the test, the multimeter is set to the ohmmeter setting and used to measure resistance across the field coils - a very high resistance may indicate an open circuit, while a very low resistance (close to zero) might suggest a short to the ground in the coil. The voltmeter function of the multimeter can also be used to ensure no voltage is present in the coils before conducting resistance tests. Tools like a soldering iron or power drill are not applicable for this test as they serve different purposes such as soldering and drilling respectively.