The correct answer is: E. No error
The sentence “Scarcely had he gone when a policeman knocked at the door” is grammatically correct. The word “scarcely” means “almost not at all” or “just before.” In this sentence, it is used to indicate that the man had just left when the policeman arrived. The word “had” is the past participle of the verb “to have,” and it is used here to form the perfect tense. The word “gone” is the past participle of the verb “to go,” and it is used here to indicate that the man had left. The word “when” is a subordinating conjunction that indicates that the action of the second clause (the policeman knocked at the door) happened after the action of the first clause (the man had gone). The word “a policeman” is the subject of the second clause, and the word “knocked” is the verb. The word “at the door” is the object of the verb “knocked.”
The sentence is grammatically correct because it follows the rules of English grammar. The words are in the correct order, and the verb tenses are correct. The sentence also makes sense, and it is easy to understand.