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

Answer: 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