Step 1: Note the misleading name.
Despite the name, ringworm has nothing to do with an actual worm, it describes the ring shaped rash the infection leaves on skin.
Step 2: Identify the true cause.
The infection, called tinea or dermatophytosis, is caused by keratin feeding fungi from genera like Trichophyton, Microsporum and Epidermophyton.
Step 3: Rule out the other pathogen types.
Viruses, bacteria and protozoans cause very different classes of disease and none of them produce this particular scaly ring shaped skin lesion.
Final answer: Option 3, fungi.