The correct answer is option 4: a combined vaccine. The DPT vaccine is a combination vaccine that provides protection against three different diseases: diphtheria, pertussis (whooping cough), and tetanus. It contains multiple components, each targeting a specific pathogen. For diphtheria, the vaccine contains an inactivated form of the toxin produced by the bacterium that causes the disease. For pertussis, it contains components from the bacterium Bordetella pertussis, which causes the infection. And for tetanus, it includes tetanus toxoid, which is a modified toxin produced by the bacterium Clostridium tetani. By combining these components into a single vaccine, it simplifies the immunization process and offers protection against multiple diseases in a single dose. Thus, DPT vaccine is categorized as a combined vaccine.