Which of the following terms would best describe an item as being of substandard quality or function?

The term "defective" is best used to describe an item that is of substandard quality or function. This word specifically refers to something that is flawed or has a fault that affects its intended use or performance. When an item is labeled as defective, it indicates that it does not meet established standards or expectations, which is central to evaluating quality in manufacturing and product design.

In contrast, the other terms convey positive attributes. "Durable" suggests that an item is well-made and able to withstand wear and tear over time, promising longevity. "Competent" typically describes a capability or skill level, indicating that something performs adequately but does not imply any deficiencies. "Efficient" refers to the effectiveness of a process in terms of time or resources, denoting optimal performance rather than issues of quality. Thus, "defective" accurately captures the essence of substandard quality or function.