In the following question, out of the four alternatives, select the alternative which is the best one word substitute for the phrase: A speech or a presentation made without previous preparation.

Introduction / Context:
This one word substitution question concerns types of speech. The phrase describes a situation where a person gives a speech or presentation without any prior preparation or written notes. This is a common concept in public speaking and debate, and English has a standard term for it. Your task is to select that single word from the options.

Given Data / Assumptions:
- Phrase: A speech or a presentation made without previous preparation. - Options: Euphemism, Obituary, Extempore, Soliloquy. - We are looking for a noun or adjective commonly used to describe unprepared speech.

Concept / Approach:
Extempore (often used as an adverb or adjective, as in speaking extempore or an extempore speech) refers to speaking without pre planning or rehearsal, relying on one's knowledge and quick thinking. The other options describe very different types of language use: euphemism is a mild expression used instead of a harsh one, obituary is a notice of someone's death, and soliloquy is a speech by a character in a play, typically alone on stage, expressing inner thoughts.

Step-by-Step Solution:
Step 1: Restate the phrase: a speech that is delivered on the spot, without any written script or advance preparation. Step 2: Examine Extempore. Extempore directly refers to speaking without preparation and is used exactly in exam contexts such as extempore speaking competitions. Step 3: Examine Euphemism. Euphemism is a mild or indirect word used instead of one that might be harsh, not a type of speech situation. Step 4: Examine Obituary. Obituary is a published notice of a person's death, sometimes including a brief biography. Step 5: Examine Soliloquy. Soliloquy is a type of dramatic monologue, usually prepared and scripted, where a character speaks internal thoughts aloud, often on stage.

Verification / Alternative check:
Use the correct word in a sentence: The candidate was asked to give an extempore speech on environmental conservation. This sentence clearly indicates that the speech is spontaneous and unprepared. Replacing extempore with obituary, euphemism or soliloquy would either make no sense or drastically alter the meaning. For instance, an obituary is written, not a spontaneous speech, and a soliloquy is carefully scripted in drama. This confirms that extempore is the intended one word substitute.

Why Other Options Are Wrong:
Euphemism is wrong because it refers to a choice of mild words, not to a mode of delivering a speech without preparation. Obituary is wrong because it is a written or printed notice concerning someone's death, not a spontaneous talk. Soliloquy is wrong because it describes a planned and scripted dramatic speech, usually delivered alone on stage in a play.

Common Pitfalls:
Students sometimes confuse extempore with soliloquy because both involve a single speaker. However, the key difference is preparation and context. Extempore is about spontaneity and lack of preparation in real life speaking, whereas soliloquy belongs to theatre and literature as a carefully written device. To remember this, associate extempore with debate competitions and interviews, where candidates are asked to speak without prior notice on a topic.

Final Answer:
The best one word substitute for “A speech or a presentation made without previous preparation” is Extempore.