**How long has Dan been driving?**

To answer this question correctly, we need to understand the tense and grammar used in the sentence. The correct answer is "has Dan been driving." Let's break down the sentence and explain why this is the correct answer.

**Understanding the sentence:**
The sentence is asking about the duration or length of time that Dan has been driving. It implies that Dan started driving at some point in the past and has continued doing so up until the present moment. Therefore, we need to use the present perfect continuous tense to convey this idea.

**Present Perfect Continuous Tense:**
The present perfect continuous tense is formed by using the auxiliary verb "has" or "have" (depending on the subject) followed by the past participle of the main verb, which is "been," and then the present participle of the main verb, which is "driving" in this case.

**Breaking down the answer:**
The correct answer is "has Dan been driving," which follows the present perfect continuous tense structure. Here's a breakdown of the components:

- "Has" is the auxiliary verb used with the third-person singular subject "Dan."
- "Dan" is the subject of the sentence, referring to the person who is driving.
- "Been" is the past participle of the auxiliary verb "be," used to form the present perfect continuous tense.
- "Driving" is the present participle of the main verb, indicating the action of driving.

**Conveying the meaning:**
By using the present perfect continuous tense, the answer conveys that Dan started driving in the past and has been driving up until the present moment. It suggests that the action of driving is ongoing and hasn't stopped yet.

In summary, the correct answer "has Dan been driving" uses the present perfect continuous tense to indicate the duration of time that Dan has been driving, implying an ongoing action from the past until the present.