Tetanus, also known as lockjaw, is caused by the bacteria Clostridium tetani. This bacterium is commonly found in the soil and the intestines of animals. When a wound becomes contaminated with soil or animal feces that contain the bacteria, it can enter the body and produce a toxin called tetanospasmin. This toxin affects the nervous system and causes muscle stiffness and spasms.

Option 1, Clostridium, is the correct answer because it correctly identifies the bacteria responsible for causing tetanus. Clostridium tetani is a spore-forming bacteria that thrives in low-oxygen environments, making it particularly dangerous in deep puncture wounds or contaminated surgical sites.

Options 2, 3, and 4 are incorrect because tetanus is not caused by a virus, bacteriophage, or salmonella. It is important to understand the correct cause of tetanus to ensure proper prevention and treatment.