Given below are four sentences, out of which one sentence is grammatically incorrect. Choose the option which points out the grammatically incorrect sentence. If all the sentences are correct, then choose ‘No error’ as the answer.

The correct answer is "She has been appointed to this post since last Monday."

Detailed Explanation:

Let's analyze the sentence to understand why it is grammatically incorrect:

1. Understanding the Action and Tense Usage:
The verb "appointed" refers to a specific event or point action that occurs at a single point in time (i.e., a completed event). Once a person is appointed, the process/event of appointing is complete.

2. The Error with "since":
The preposition "since" is used with perfect continuous or present perfect tenses to denote a state or action that started in the past and continues up to the present moment (e.g., "She has been working here since last Monday").
However, an event that happens at a specific instant (like an appointment) cannot be ongoing continuously over a duration from "last Monday" to the present. To indicate when a specific past event took place, we use simple past tense with a time expression (e.g., "was appointed on last Monday" or "was appointed last Monday").

3. How to Correct the Sentence:
- If referring to the event: "She was appointed to this post last Monday."
- If referring to the duration of holding the post: "She has held this post since last Monday." or "She has been in this post since last Monday."

Analysis of Other Options (Which are all grammatically correct):

- "The warden warned the inmates to follow the rules strictly." — Correct usage of simple past tense and reporting verb structure.
- "He declined to comment on the matter during the hearing." — Correct infinitival construction ("declined to comment").
- "The letter was signed and dispatched before sunset." — Correct passive voice usage in the simple past tense.