Out of the four alternatives, choose the one word that can be substituted for the phrase "of the highest quality". This tests your understanding of vocabulary related to very high standards and excellence.

Introduction / Context:
This question is a one-word substitution problem focusing on vocabulary used to describe quality and standards. The phrase "of the highest quality" is common in advertisements, reviews, and academic writing. You are asked to choose the single word that best captures the sense of something being of the very best or highest degree when compared with others.

Given Data / Assumptions:

We need a word that expresses the idea of the very best or highest degree of quality.
The context is general English usage, including grammar and descriptive writing.
Some options are unrelated to quality and instead relate to production or reaction.
The correct option must clearly indicate excellence or the highest level.

Concept / Approach:
In English grammar and general vocabulary, the word superlative is used to refer to the highest degree of comparison, as in tallest or most intelligent. Outside grammar, superlative can also describe something that is of the highest possible quality. For example, we may speak of superlative workmanship or superlative performance, meaning extremely good or the best. Productive and reactive relate to output or responsiveness, relative indicates comparison but not necessarily the highest level, and moderate suggests average rather than the best. Therefore, superlative is the only option that matches the phrase given.

Step-by-Step Solution:
Step 1: Focus on the core idea: "of the highest quality" means better than all others or at the maximum level of excellence. Step 2: Recall that superlative is used for the highest degree of comparison and is also used more generally to mean excellent or of the highest quality. Step 3: Examine option C Superlative and see that it directly aligns with this concept. Step 4: Consider option A Productive, which means producing a lot or being efficient, not necessarily of the highest quality. Step 5: Recognise that option B Reactive describes how something responds to stimuli, not how good the quality is. Step 6: Options D Relative and E Moderate both suggest comparison or middle level, not excellence or the top position.

Verification / Alternative check:
Use the words in example sentences. A superlative performance by the athlete impressed everyone clearly suggests the very highest quality. In contrast, a productive performance sounds odd and does not clearly mean highest quality; it would refer more to output or quantity. Similarly, a moderate performance suggests average ability, not excellence. Observing how these words are used in real contexts confirms that superlative is the only correct answer when we want to describe something as being of the highest quality.

Why Other Options Are Wrong:
Option A Productive focuses on output or usefulness, but something can be productive without being the very best in quality.
Option B Reactive means responsive to changes or stimuli and has nothing to do with quality levels.
Option D Relative indicates something that is considered in relation to something else; it does not indicate the top level or highest quality.
Option E Moderate describes a middle or average level, the opposite of what is suggested by the phrase of the highest quality.

Common Pitfalls:
Students sometimes confuse words connected with comparison. While comparative and relative suggest some kind of comparison, they do not guarantee that something is the best. Superlative is the term reserved for the highest degree among three or more items. Remember that in grammar we say positive, comparative, superlative, and that the superlative form represents the topmost degree. This memory aid will help you in both grammar and vocabulary questions like this one.

Final Answer:
The one word that means "of the highest quality" is superlative.