The correct answer is option 4, which is bacterial. Typhoid fever is caused by the bacteria called Salmonella Typhi. These bacteria are transmitted through contaminated water or food. Algae, option 1, are not known to cause typhoid fever. Parasites, option 2, are organisms that live in or on another organism and obtain nutrients from it, but they are not responsible for causing typhoid fever. Protozoa, option 3, are single-celled organisms that can cause various diseases, but they are not the cause of typhoid fever. It is important to note that the correct answer may vary depending on the context and specific circumstances, but in the case of typhoid fever, it is caused by a bacterial pathogen.