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

A common sign of food spoilage is an unpleasant odor. When food spoils, bacteria and other microorganisms break down the food's components, producing gases and other byproducts that often result in a foul smell. This distinctive odor signals that the food is no longer safe to eat, as it indicates the presence of harmful microorganisms or the onset of decay.

In contrast, intact packaging generally indicates that food is preserved and less likely to be spoiled. Bright colors can be an indication of freshness in fruits and vegetables, and while cooling temperatures can slow down spoilage, they do not inherently indicate the state of the food itself. Thus, the presence of an unpleasant odor is a reliable indicator of compromised food quality and safety.