To the mother’s surprise, the soup had gone completely mouldy.

The correct answer is A.

Mouldy is a word that describes something that has been covered in a fungus that grows in damp places. It is often green or black in colour and can be harmful to humans if eaten.

The other options are not correct because they do not describe the soup in the same way.

• Finished means that the soup has been eaten or used up.
• Cold means that the soup is not hot.
• Stale means that the soup is no longer fresh and has a bad taste.
• Sour means that the soup has a sour taste, like vinegar.