Explanation:

The correct answer is "had been standing". This is because the sentence is in the past perfect continuous tense, which is used to describe an action that started in the past and continued up until another action in the past.

Breaking down the sentence:

- Subject: A young boy
- Verb: stand
- Tense: Past perfect continuous
- Object: None
- Additional information: Until a policeman arrived

Why past perfect continuous tense?

The past perfect continuous tense is used to describe an action that started in the past and continued up until another action in the past. In this sentence, the young boy was standing at the door until the policeman arrived. The action of standing started in the past and continued until the policeman arrived, which is also in the past. Therefore, the past perfect continuous tense is the correct tense to use in this sentence.

Forming the past perfect continuous tense:

The past perfect continuous tense is formed by using the auxiliary verb "had" in the past perfect tense, the main verb in the present participle form (-ing), and the word "been". For example, "I had been working for six hours before I took a break."

In this sentence, "had been standing" is the correct form of the past perfect continuous tense for the verb "stand". Therefore, the correct sentence is "A young boy had been standing at the door until a policeman arrived."