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 requested him that he should explain the procedure again."

Detailed Explanation & Logical Analysis:

1. Analyzing the Incorrect Sentence:
In English grammar, the verb "request" when used to report a imperative request or command takes a direct object followed by an infinitive structure (to + base verb), i.e., request + object + to-infinitive.
Alternatively, if a 'that'-clause is used after verbs of request, suggestion, or command (subjunctive mood), it follows the pattern: request + that + subject + base form of verb (without "should").
Using "requested him that he should..." creates redundancy and incorrect structural usage because "requested" already implies a modal sense of asking/urging someone to do something.

Correct Formats:
• Infinitive pattern: "She requested him to explain the procedure again."
• Subjunctive pattern: "She requested that he explain the procedure again."

2. Verification of Other Options:
• "The constable asked the visitors to wait outside the gate." — Grammatically correct (uses standard reporting verb 'asked' + object + to-infinitive).
• "The luggage was checked thoroughly before boarding the train." — Grammatically correct (uses correct passive voice structure and adverb usage).
• "He arrived on time and took his seat without any delay." — Grammatically correct (uses parallel past tense verbs 'arrived' and 'took').

Hence, the sentence "She requested him that he should explain the procedure again." contains grammatical error.