In the following English vocabulary question on idioms, choose the alternative that best expresses the meaning of the phrase "of the first water", which is often used to describe the exceptional quality or purity of something or someone.

Introduction / Context:
The idiom "of the first water" comes originally from the world of gemstones, especially diamonds, where "water" referred to clarity and brilliance. Today it is used more generally to describe something of the highest quality, excellence, or purity, whether it is a talent, character, or skill. This question checks whether you can connect such a traditional idiom with its correct modern meaning, a common task in English sections of competitive exams.

Given Data / Assumptions:

• The idiom given is "of the first water".
• You must choose the option that accurately gives its meaning.
• The idiom is positive and refers to quality or excellence.
• No additional sentence context is provided, so you rely on vocabulary knowledge.

Concept / Approach:
Originally, jewelers used "first water" to describe diamonds with the greatest purity and brilliance. Over time, the expression has been extended to situations where someone wants to describe a person or thing as being of the very finest kind, such as "a musician of the first water" or "honesty of the first water". Therefore, we look for an option that clearly indicates highest quality rather than comfort, importance, or conflict of interest. Matching the emotional tone and historical origin of an idiom leads you to the right answer.

Step-by-Step Solution:
Step 1: Recall that "of the first water" is used in a strongly positive way to praise someone or something. Step 2: Connect "first water" with the idea of the purest, most flawless diamond, which implies the very best quality. Step 3: Examine the options and eliminate meanings that do not talk about quality or excellence. Step 4: Notice that Option D, "Of the best quality", exactly summarises this sense of highest excellence. Step 5: Confirm that the remaining options either introduce unrelated ideas (comfort, importance, acting against interests) and therefore are unsuitable.

Verification / Alternative check:
A quick mental check using example sentences helps. You could say "Her integrity is of the first water" or "He is a leader of the first water." In each case, replacing "of the first water" with "of the best quality" keeps the meaning intact, whereas replacing it with "leading a comfortable life" or "critically important" sounds wrong. This confirms Option D as the correct interpretation.

Why Other Options Are Wrong:

• Option A, "Leading a comfortable life", describes lifestyle, not excellence, and does not relate to the imagery of clarity or quality.
• Option B, "Acting contrary to the ones interests", describes self harming behaviour and has a negative sense, unlike this positive idiom.
• Option C, "Critically important", is about significance or priority, not about purity or the best standard.

Common Pitfalls:
Students sometimes confuse positive idioms with general words for importance or success, selecting options about being critical or vital instead of quality. Others may misinterpret "water" literally and think of comfort or basic needs. To avoid such errors, remember that many idioms have historical roots in specific professions, like jewelry in this case, and their modern meanings still reflect that origin. Always check whether the option matches both the emotional tone and the underlying image of the idiom.

Final Answer:
The idiom "of the first water" means that something is of the very highest quality, so the correct option is Of the best quality.