The correct answer is option 3: Virus. The common cold is primarily caused by viral infections. Viruses, specifically rhinoviruses, are responsible for causing most cases of the common cold. These viruses can easily spread through droplets in the air when an infected person sneezes or coughs, or by touching contaminated surfaces and then touching the face.

Option 1, bacteria, is not the cause of the common cold. Bacterial infections, such as strep throat, can have similar symptoms to the common cold, but they are caused by different microorganisms.

Option 2, fungi, is also not the cause of the common cold. Fungal infections typically affect areas like the skin or nails, and they are not associated with the common cold.

Option 4, protozoa, are single-celled organisms that can cause various types of infections, but they are not known to cause the common cold.

In summary, the common cold is primarily caused by a virus, specifically rhinoviruses.