Explanation:

Present Perfect Tense:
The correct answer to the question "________ (you buy) your new car yet?" is "Have you bought." This is because the question is asking about a past action that has a connection to the present. In English, we often use the present perfect tense to talk about actions or events that happened in the past but have a relevance or impact on the present.

Forming the Present Perfect:
To form the present perfect tense, we use the auxiliary verb "have" (in the appropriate form) followed by the past participle of the main verb. In this case, the main verb is "buy," and the past participle is "bought."

Subject-Verb Agreement:
In the given question, the subject is "you." Since "you" is a second-person pronoun, the correct form of the verb is "have." This is because "have" is the correct second-person singular and plural form of the auxiliary verb "to have" in the present perfect tense.

Asking about the Action:
In the question, the word "yet" indicates that the action of buying the new car is expected or anticipated to have happened already. The use of "yet" suggests that the speaker is curious to know if the action has been completed or not.

Alternative Incorrect Answers:
- "Did you buy?" - This answer is incorrect because it uses the simple past tense ("did") instead of the present perfect tense. The simple past tense is used to talk about completed actions in the past with no connection to the present. However, in the given question, we are asking about a past action that has a relevance to the present.
- "Do you buy?" - This answer is incorrect because it uses the present simple tense ("do") instead of the present perfect tense. The present simple tense is used to talk about general habits or routines, not specific past actions.

Conclusion:
The correct answer to the question "________ (you buy) your new car yet?" is "Have you bought." This answer uses the present perfect tense to indicate a past action with a connection to the present. It is grammatically correct and aligns with the rules of subject-verb agreement.