Which word describes a quality that is lacking or insufficient for a purpose?

The term "inadequate" specifically refers to something that does not meet the required standards or needs for a particular purpose. It suggests a quality or quantity that falls short of what is necessary, making it directly relevant in contexts where a minimum threshold must be met.

While "insufficient" and "deficient" can convey similar meanings, "inadequate" emphasizes the insufficiency in a more qualitative way, aligning closely with situations where something does not meet expectations or requirements. "Scarce," on the other hand, relates more to availability rather than the quality or effectiveness of a resource. Therefore, the emphasis on failure to meet a standard with "inadequate" solidifies its appropriateness as the correct choice.