The presence of pus indicates:

The presence of pus is a key indicator of an infection, particularly bacterial in nature. Pus is a thick fluid that is typically yellow or green in color and is composed of dead white blood cells, bacteria, and tissue debris. Its formation occurs as part of the body’s immune response to a pathogenic invasion, where the immune system sends white blood cells to the site of infection to fight off the harmful microbes.

While an allergic reaction can cause various skin symptoms, it typically does not lead to the formation of pus. Similarly, a viral infection usually does not produce pus; instead, viral infections often trigger other types of immune responses without creating the tissue breakdown associated with pus formation. While it's important to note that pus is primarily associated with bacterial infections, it can sometimes be present in mixed infections involving both bacteria and viruses. However, the definitive link of pus to the presence of an infection makes the answer "B" the most accurate choice in this context.