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/answer is: He has completed the task and left the office since noon.

Explanation:

Let us analyze the grammatical accuracy of each sentence to understand why this sentence is incorrect:

1. "He has completed the task and left the office since noon."
This sentence contains a grammatical error regarding the usage of the present perfect tense with the preposition "since".

The word "since" is used to indicate a time frame extending from a specific point in the past up to the present moment. However, the action of leaving ("left") is a single, completed event in the past (simple past tense), not a continuous state or repeated action extending to the present.

To fix the sentence, we should either use "at" to specify the exact time of leaving, such as:
"He completed the task and left the office at noon."
Or revise it to reflect duration up to the present:
"He has been away from the office since noon."

2. Let's look at the remaining sentences:

• "The warden instructed the staff to maintain strict discipline." — Grammatically correct. Follows standard subject-verb-object structure with an infinitive phrase (to maintain...).

• "The notice was posted on the board for everyone to read." — Grammatically correct passive voice construction.

• "She spoke briefly but made her point very clearly." — Grammatically correct usage of adverbs modifying verbs (spoke briefly, made... clearly).