Choose the correct answer. If he is thirsty, he should buy a ... of water.

Explanation:

• The question asks what someone should buy if they are thirsty and want water.
• The options provided are 'bottle', 'cup', 'carton', and 'can'.
• While water can come in cups, cartons, or cans, the most common and direct way to purchase water for immediate consumption when thirsty is often a bottle.
• The phrase "buy a ... of water" suggests a container that is typically purchased as a unit.

Answer: bottle