The correct option is C bacteria
Bacteria is the causative agent of tetanus.
Tetanus is a nervous disease caused by a toxin producing bacterium called Clostridium tetani.
Tetanus disease causes muscle contractions particularly of jaw and neck muscles; commonly known as lockjaw. Severe complications of this disease can be life-threatening. The bacterium normally survives in dormant state until they get a place to flourish, like a wound- a condition good for growth and reproduction. As they grow and divide, they release a toxin called ‘tetanospasmin’, this toxin impairs the nerves in the body that controls the muscle.
Protozoa are microscopic, single celled eukaryotic organisms, protozoans are either parasites or free-living. There are many protozoans that are infectious and cause various diseases like Plasmodium, Entamoeba, etc.
Fungi are a group of saprotrophic and parasitic spores producing eukaryotic organisms. Many of the fungi species are infectious and cause disease like ringworm, athlete's foot, etc.
Virus are the non-living infectious agent, generally a small collection of genetic code, either DNA or RNA, surrounded by a protein coat, incapable of replicating in itself, it only replicates and spreads once it infects a host cell.