Which of the following is a sign of food spoilage?

An unusual odor is a key indicator of food spoilage due to the growth of microorganisms such as bacteria and molds. When food decomposes, these microorganisms produce byproducts that can emit strong or off-putting smells. This change in odor signals that the food may not be safe to consume, as it indicates that the quality has degraded and it could harbor harmful pathogens or toxins. Recognizing an unusual odor is vital for maintaining food safety and preventing foodborne illness.

In contrast, bright color can often be a sign of freshness, reflecting the food's quality rather than spoilage. Perfectly sealed packaging usually indicates that the food is protected from contaminants, thus reducing the chances of spoilage. Similarly, a fresh appearance generally suggests that the food is still good to eat. Therefore, the presence of an unusual odor is a definitive sign that food has spoiled and may pose health risks.