The principle defines property as consisting of the right to possess, use, alienate (transfer), and exclude others, and says a sale is complete once these rights transfer from seller to buyer. A sold the car to B, and B then asked A to physically keep the car for a month on B's behalf, which A agreed to. Let's check each option against this principle.
Since the facts describe the sale as already having taken place, with A retaining the car afterward only as a caretaker on B's behalf, the sale of the car is complete.
Therefore, the correct answer is sale of car is complete.
The principle sets up a conditional test: property consists of the rights to possess, use, alienate and exclude others, and a sale is complete precisely when this bundle of rights passes from seller to buyer, regardless of who physically holds the item afterward. Applying this test to the facts, the facts state that A sold his car to B, and only afterward does B ask A to keep the car in his care for a month. Let's test each option against this rule.
Since the facts place the transfer of ownership rights before the custody arrangement, the rule's condition for a complete sale is already satisfied, and the later care arrangement does not undo it.
Therefore, the correct answer is sale of car is complete.