In the following English vocabulary question, you are given a descriptive phrase. Choose, from the four alternatives, the one word that best substitutes the phrase "too extreme, vague or indefinite to be adequately described in words".

Introduction / Context:
This one word substitution item tests your ability to choose the correct adjective for something that cannot be adequately described. The phrase given is extreme or indefinite to be adequately described, which clearly points towards a word meaning beyond description. You have to select the option that naturally expresses this idea in standard English.

Given Data / Assumptions:

• The phrase suggests that the thing is too extreme, vague or indefinite to be fully expressed in words.
• Option A: Ostensible.
• Option B: Indescribable.
• Option C: Impressionable.
• Option D: Conceivable.
• We assume that one of these adjectives directly conveys the sense of not being describable.

Concept / Approach:
The most direct and accurate word for something that cannot be properly put into words is indescribable. It means beyond description or impossible to describe adequately. Ostensible means apparent or seemingly true but not necessarily so. Impressionable describes someone who is easily influenced, and conceivable means that something can be imagined or thought of. Of these, only indescribable fits the idea of being too extreme or indefinite to be described.

Step-by-Step Solution:
Step 1: Focus on the key idea: the thing described in the phrase is beyond adequate description in words.Step 2: Recall the meaning of indescribable, which is used for experiences, feelings or beauty that words fail to capture fully.Step 3: Match this meaning with option B, indescribable, which literally contains describe and the negative prefix in.Step 4: Examine ostensible, which refers to something that appears to be real or true but may hide a different reality; it does not convey being beyond description.Step 5: Reject impressionable and conceivable as they describe a person or the possibility of imagining something, not the inability to describe it. Therefore, indescribable is the correct choice.

Verification / Alternative check:
Test the options in sample sentences. You can say The view from the mountain was indescribable, meaning that it was so beautiful that words were not enough. If you substitute ostensible, impressionable or conceivable, the sentence either becomes incorrect or changes meaning completely. The phrase an indescribable feeling is common in English and matches well with the idea in the stem, confirming that indescribable is the right answer.

Why Other Options Are Wrong:
Ostensible is used in contexts such as the ostensible reason for the visit, meaning the apparent or stated reason, not something beyond description. Impressionable applies to people, especially young people, who are easily influenced by others. Conceivable describes something that can be imagined or believed, often used in phrases like every conceivable option. None of them relates to the inability to describe something due to its extremity or vagueness.

Common Pitfalls:
Because indescribable directly contains describe, some test takers mistakenly think it is too obvious to be correct and start considering more complex sounding options. However, exam designers often include the straightforward choice. A good approach is always to match the core meaning first rather than overcomplicating. When the question clearly points to beyond description, indescribable is exactly the word that should come to mind.

Final Answer:
The correct one word substitute is Indescribable, so option B is correct.