Explanation:

The correct answer is "has been standing."

Present Perfect Continuous Tense
The sentence describes an action that started in the past and is still ongoing in the present. The present perfect continuous tense is used to express this kind of action.

Formation of the Present Perfect Continuous Tense
The present perfect continuous tense is formed using the auxiliary verb "has/have" in the present perfect tense, followed by "been" and the present participle ("-ing" form) of the main verb.

Examples:
- I have been waiting for the bus.
- She has been studying all night.

Explanation of the Given Sentence:

The sentence states that a little boy has been standing at the door since noon. Let's break down the sentence to understand it better.

Subject: A little boy
Verb: has been standing
Adverbial Phrase: since noon

The subject of the sentence is "a little boy," and the verb phrase is "has been standing."

Present Perfect Tense:
The use of "has" indicates that the action is being performed by a singular subject (the little boy) in the present perfect tense.

Continuous Aspect:
The use of "been standing" indicates that the action is continuous and ongoing. The present participle form ("-ing" form) of the main verb "stand" is used in conjunction with the auxiliary verb "has been."

Adverbial Phrase:
The phrase "since noon" provides additional information about when the action started. It indicates that the little boy started standing at the door at noon, and he is still standing there at the time of speaking.

Conclusion:
By using the present perfect continuous tense ("has been standing"), the sentence effectively conveys that the little boy started standing at the door at noon and has continued to do so until the present moment.