Final answer:
The correct representation of the transaction where Donnie sold his car for $4000 is a positive integer, showing the income he received, thus the answer is A. $4000.
Step-by-step explanation:
Representing the situation with an integer where Donnie sold his car to his next-door neighbor for $4000, we should consider what this transaction means for Donnie's financial status. Since Donnie is selling the car and receiving money in this transaction, the correct representation of the situation would be a positive income for Donnie. Therefore, the answer should be a positive integer. In this case, the best choice is A. $4000 as it shows the amount Donnie gained from the sale.