Which of the following is a common sign of food spoilage?

A foul odor is a clear indicator of food spoilage because it often results from the breakdown of food compounds and the growth of bacteria or mold. When food spoils, it undergoes chemical changes and microbial activity, leading to the production of volatile compounds that emit unpleasant smells. This is usually a result of protein decomposition and is a warning sign that the food is no longer safe to consume.

In contrast, bright colors typically indicate freshness, as vibrant colors in fruits and vegetables are often associated with ripeness and good quality. A dry texture may signal dehydration or loss of moisture, which can sometimes occur in food but isn't a definitive sign of spoilage. A fresh appearance, with intact texture and appealing colors, usually suggests that the food is still good to eat. Thus, the presence of a foul odor uniquely signifies spoilage among the options provided.