Which word describes a blend that results in a deterioration of quality?

The most suitable word to describe a blend that results in a deterioration of quality is "alloy." An alloy is formed when two or more elements, typically metals, are combined to improve certain properties; however, in some contexts, the blending of less desirable materials can lead to a reduction in the overall quality or performance of the resulting compound.

In metallurgy, for example, if a quality metal is blended with a cheaper, lower-grade metal, the final product may exhibit diminished strength or durability, thus illustrating the deterioration in quality that can occur from the blending process.

The other options do not convey the same sense of reduction in quality. "Theft" relates to the act of stealing, which doesn't pertain to a blend or mixture. "Mixture" and "compound" describe combinations of substances without implying any negative impact on quality; they are neutral terms that do not indicate deterioration specifically. Therefore, "alloy" is the word that encapsulates the notion of a blend leading to potential degradation in quality.