The exposed milk in the container has turned …

Analyze the question: We need the word that best describes what happens to milk left exposed. This tests your knowledge of words used for food going bad, because each option has a specific usage.

Explain the concept: Different words describe different kinds of spoilage:

• Sour: having a sharp, acidic taste or smell, typical of milk that has gone bad because bacteria turn the lactose into lactic acid.
• Putrid: rotten and foul-smelling, usually for decaying flesh or organic matter (meat, fish, corpses).
• Mouldy: covered with mould (fungus), typical of bread, cheese, or fruit.
• Stale: no longer fresh, usually for bread, cake, biscuits, or air. It describes dryness or lost freshness, not fermentation.

Step-by-step solution:

1. The item is milk, a liquid.
2. When milk is exposed and left unrefrigerated, bacteria ferment it and it develops a sharp taste and smell.
3. The standard word for this is sour. We say "the milk has turned sour". The phrase "turned sour" is a very common collocation.
4. Checking the others: "putrid" is too strong and is not used for milk, "mouldy" implies visible fungal growth, and "stale" is not used for liquids like milk.

Correct answer: A. sour

Tip: Memorize the common pairings: sour milk, stale bread, mouldy cheese, rotten eggs, putrid meat. In the exam, the word that collocates naturally with the noun is usually the right one.