**Explanation**

The correct answer is option A: "How many biscuits have you eaten?"

**1. Verb Tense:**
- Option A uses the present perfect tense, "have you eaten," which indicates an action that started in the past and has a connection to the present. This tense is commonly used to talk about experiences or activities that happened at an unspecified time before now.
- Option B uses the present perfect continuous tense, "have you been eating," which indicates a continuous action that started in the past and is still happening in the present. This tense is used to emphasize the ongoing nature of an action.

**2. Context:**
- Without any additional context, option A is more likely because it asks for a total or final count of biscuits consumed. It suggests that the action of eating biscuits has already been completed and the speaker wants to know the total quantity.
- Option B, on the other hand, implies that the speaker wants to know how many biscuits the person has been continuously eating, suggesting that the action is still ongoing.

**3. Common Usage:**
- In everyday conversation, it is more common to use the present perfect tense (option A) when asking about the total number of biscuits someone has eaten.
- The present perfect continuous tense (option B) is less commonly used in this context. It is generally employed when focusing on the ongoing process or duration of an action rather than the final result.

**4. Grammatical Correctness:**
- Both options A and B are grammatically correct, but option A is the better choice based on the given context and common usage.

In conclusion, option A ("How many biscuits have you eaten?") is the more likely and appropriate sentence. It asks for the total number of biscuits consumed, indicating that the action of eating biscuits has already been completed.