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.

Correct Answer: He has completed the task and left the office since noon.

Step-by-Step Explanation:

1. Analyzing the Incorrect Sentence:
The sentence "He has completed the task and left the office since noon." contains a grammatical error regarding the usage of time prepositions and tenses.

2. Why is it incorrect?
- The word "since" is used to indicate the starting point of an action that continues up to the present moment (e.g., "He has been working since noon").
- A past completed action occurring at a specific point in time (noon) should be expressed using a past time preposition like "at" or by rephrasing with the simple past tense (e.g., "He completed the task and left the office at noon").

3. Verification of Other Options:
- "The warden instructed the staff to maintain strict discipline." — Grammatically correct.
- "The notice was posted on the board for everyone to read." — Grammatically correct.
- "She spoke briefly but made her point very clearly." — Grammatically correct.

Hence, the sentence pointing out the grammatical error is "He has completed the task and left the office since noon."