6514. To the mother’s surprise, the soup had gone completely mouldy.

✅ The correct answer is option C.
Finished : (of a task or activity) brought to an end; completed.

Cold : of or at a low or relatively low temperature, especially when compared with the human body.

Stale : (of food) no longer fresh and pleasant to eat; hard, musty, or dry.

Sour : having an acid taste like lemon or vinegar.

Mouldy : covered with a fungal growth which causes decay, due to age or damp conditions.

Synonym of Mouldy is Stale