Choose the correct one word substitution for the following description: A thing that is no longer in use.

Introduction / Context:
This vocabulary question tests your knowledge of one word substitutions and your ability to distinguish between similar looking words. The phrase given is a thing that is no longer in use. In everyday and technical English, an item, idea, or system that has fallen out of use because it is old fashioned or has been replaced is called obsolete. Identifying the correct word here is essential for precise expression in writing and speaking.

Given Data / Assumptions:

The phrase is: The thing no longer in use.

The context is general and can apply to objects, methods, or ideas.

The options share a similar starting spelling ob but have different meanings.

We are looking for the word that exactly matches the idea of not being used any more.

Only one option should be fully correct.

Concept / Approach:

The important words in the phrase are thing and no longer in use. In standard English, obsolete is an adjective that means out of date, no longer used, or no longer produced. Obstacle refers to something that blocks progress, obstruction has a similar meaning of blockage, obsidian is a volcanic glass, and obligation means duty or responsibility. None of these except obsolete contains the idea of being out of use. Even though the question says the thing, exams often use the describing adjective obsolete in such one word substitution questions.

Step-by-Step Solution:
Step 1: Focus on the key idea: something that used to be used but is not used now. Step 2: Examine option a, obstacle. This is anything that blocks progress or makes movement difficult; it does not mean out of use. Step 3: Examine option b, obsolete. Dictionaries define obsolete as no longer produced or used, out of date. This exactly fits the phrase. Step 4: Examine option c, obsidian. This is a type of natural volcanic glass, often black and shiny; it has no connection with usage. Step 5: Examine option d, obstruction. This is again something that blocks or prevents passage, like an obstacle. Step 6: Examine option e, obligation. This refers to a moral or legal duty, not the state of an item being outdated. Step 7: Select option b, obsolete, as the only correct match.

Verification / Alternative check:

We can compare with example sentences: Many old computer systems have become obsolete or The cassette tape is now considered obsolete technology. In both cases, obsolete describes things that are no longer in common use. Substituting obstacle, obstruction, or obligation would not make sense in these sentences. This confirms that obsolete captures the intended meaning.

Why Other Options Are Wrong:

Option a, obstacle, focuses on a barrier to progress, not on being outdated.

Option c, obsidian, names a specific natural material and is unrelated to usage or modernity.

Option d, obstruction, again means blockage and does not express the idea of something having fallen out of use.

Option e, obligation, expresses duty or something you are required to do, which is very different from the state of an unused object.

Common Pitfalls:

A common trap in such questions is to be guided by the first few letters of the word rather than the full dictionary meaning. Because obstacle, obstruction, and obsolete all begin with ob and may have been encountered by students in similar contexts, they may be confused. The correct strategy is to focus on the exact phrase no longer in use and recall that obsolete is the standard technical and everyday term for this concept.

Final Answer:

The correct one word substitution is Obsolete.