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 option pointing out the grammatically incorrect sentence is:
She has been appointed to this post since last Monday.

Explanation:
Let us analyze the grammatical structure of the sentence:

1. The sentence uses the present perfect passive form "has been appointed".
2. "Appointed" refers to a point action (a single event that happened at a specific time in the past), not a continuous duration or a state extending from the past to the present.
3. Since the action of being appointed occurred at a specific time in the past ("last Monday"), the simple past tense should be used instead of the present perfect continuous/perfect state with "since".
4. The correct sentence structure should be: "She was appointed to this post last Monday." (or if referring to duration in office: "She has been in this post since last Monday.").

Now let us evaluate the other options:
- "The warden warned the inmates to follow the rules strictly." — Grammatically correct (uses correct past tense and infinitive structure).
- "He declined to comment on the matter during the hearing." — Grammatically correct.
- "The letter was signed and dispatched before sunset." — Grammatically correct (simple past passive voice).

Therefore, the sentence containing a grammatical error is indeed "She has been appointed to this post since last Monday."