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.)

Yes, you're correct! The answer is 'carton'. Packet is for loose food, box is for large, loose items and bar is like a chocolate bar! You could also say, I would like a PINT of milk. That's if you want to use quantities of course :) *following

тільки carton

Carton. Since milk is mostly likely to come in a carton.