Which of the following is NOT a restriction on the use of aluminum conductors?

The choice made regarding bare exposed concrete as NOT being a restriction on the use of aluminum conductors is correct because aluminum conductors can typically be utilized in various settings including being in contact with bare concrete, as long as they are properly insulated and specified for use. Concrete does not introduce significant electrochemical corrosion risk to aluminum, especially if proper installation practices are followed.

On the other hand, specific environments like wet areas, high-tension applications, and enclosed spaces often impose restrictions on the use of aluminum conductors due to the material's susceptibility to corrosion and other environmental factors when not properly handled. In wet environments, aluminum can corrode, especially if moisture leads to galvanic action when it contacts dissimilar metals. Under high tension, the physical limitations of aluminum compared to copper make it less suitable unless properly rated. In enclosed spaces, concerns over temperature rise and potential for fire hazards can also influence the choice of conductor material and its installation method.