In the question below, four sentences have been given. Out of four sentences, one sentence is error-free. Find out which of the following sentence is error-free. If all the given sentences have errors, then choose option ‘None is correct’ as the answer choice.

The correct answer is "Scarcely had the rebels laid down arms when fresh skirmishes erupted near the northern border."

Let's analyze each sentence step-by-step to identify the grammatical errors and find the error-free option:

1. "Despite his best efforts, the conclusion he arrived lacked coherence and failed to address the premise."
* Error: Missing preposition.
* Correction: The verb "arrive" requires the preposition "at" when referring to reaching a conclusion. It should be: "the conclusion he arrived at lacked coherence..." or "the conclusion at which he arrived lacked coherence..."

2. "Scarcely had the rebels laid down arms when fresh skirmishes erupted near the northern border."
* Analysis: This sentence is grammatically correct.
* It correctly uses the correlative conjunction structure "Scarcely... when".
* Since the sentence begins with the negative/restrictive word "Scarcely", subject-verb inversion is required: the auxiliary verb "had" correctly comes before the subject "the rebels" ("had the rebels laid down").
* The past participle "laid" (from the transitive verb "lay", meaning to put down) is also correctly used here with the direct object "arms".

3. "Whosoever they appoint for the post must demonstrates both integrity and strategic foresight."
* Error 1: Incorrect verb form after a modal verb.
* Correction 1: The modal auxiliary verb "must" must be followed by the base form of the verb, which is "demonstrate", not the third-person singular "demonstrates".
* Error 2: Incorrect pronoun case.
* Correction 2: Since the pronoun is the object of the verb "appoint" (i.e., they appoint him/her), it should be in the objective case: "Whomever" (or "Whomsoever"), rather than the subjective case "Whosoever". The corrected segment is: "Whomever they appoint... must demonstrate..."

4. "Not until the investigation was completed we knew the extent of the financial discrepancies."
* Error: Missing subject-auxiliary inversion.
* Correction: When a sentence begins with a negative adverbial clause like "Not until...", the main clause must undergo subject-auxiliary inversion. Therefore, "we knew" must be inverted using the auxiliary verb "did". It should be: "Not until the investigation was completed did we know the extent..."