Only power service-drop or overhead service conductors should be attached to a service ______.

The correct response emphasizes the necessity of proper installation practices concerning electrical service drops or overhead service conductors. The term "must" indicates that there is a requirement or regulation in place dictating that only power service-drop or overhead service conductors should be connected to a service. This is crucial for ensuring safety, compliance with electrical codes, and the reliable operation of electrical systems.

In this context, using "must" implies that not adhering to this guideline could result in unsafe conditions or violations of electrical standards. Regulations typically exist to protect against issues such as overloading, improper grounding, or hazardous conditions that could arise if incorrect types of conductors were utilized in the service connection.

The other choices imply varying levels of possibility or recommendation rather than a definitive requirement, which does not align with the stringent safety standards necessary in electrical installations. Thus, only "must" correctly conveys the importance and obligation of following this guideline.