Concept:The correct word is "sour", which can mean to spoil or become bad.
Explanation:"Sour" is the best choice because it describes food or drink that has turned acidic, unpleasant, or unfit to eat.
For example, milk becomes sour when it is kept for too long.
The word can also mean to become rancid, mildewed, or spoiled.
"Putrid" and "mouldy" describe different forms of decay, while "stale" means not fresh or dry.
Therefore, "sour" best fits the meaning required.
Answer:A. sour