If 15- and 20-ampere duplex receptacles are not marked with "CO/ALR," they are suitable for use with?

The correct choice indicates that 15- and 20-ampere duplex receptacles not marked with "CO/ALR" are suitable for use with copper and copper-clad aluminum conductors only. This is accurate because the marking "CO/ALR" signifies that the receptacle has been tested for safe use with aluminum conductors, particularly in terms of ensuring the connection points can handle the different expansion and contraction rates of copper and aluminum without degrading.

Without this marking, the receptacle may not adequately accommodate the properties of aluminum conductors, therefore limiting its use. Copper conductors are generally deemed safe for use with most receptacles, but the inclusion of copper-clad aluminum conductors ensures compatibility with both materials without compromising safety or performance.

Recognizing that receptacles not labeled "CO/ALR" are specifically deemed unsuitable for installations involving aluminum conductors helps reinforce the importance of understanding these markings when making wiring decisions for electrical safety and compliance.