Final answer:
The unacceptable use of a flexible cord is as a substitute for fixed wiring. They are intended for temporary power, portable equipment, and specific appliance situations, not as a permanent solution.
Step-by-step explanation:
The use for a flexible cord or cable that is not acceptable is: A) As a substitute for fixed wiring. Flexible cords and cables are designed for specific uses such as for temporary electrical power, for portable equipment connections, and sometimes for adjustments in certain appliances. However, they are not meant to replace permanent household or building wiring. The other options, such as for temporary electrical power (B) and for portable equipment connections (D), are acceptable uses. Using flexible cords in concealed locations (C) can be permissible depending on regulations and specific scenarios that ensure safety and adherence to the electrical codes.