The answer is 'carton.' Why? Because quite simply milk is sold in cartons (or bottles but that's not a listed option.)
Honestly the easiest way to see the differences between these words is to search for images, rather than have someone do a half-hearted job of explaining the differences between them. If you search for packet food, (things like crisps, biscuits, sweets etc. come in packets it will quickly become apparent to you why a liquid such as milk is not sold in packets.)