The DPT vaccine, also known as the DTaP vaccine, is given to children to immunize them against three different diseases: diphtheria, whooping cough (also referred to as pertussis), and tetanus.

Diphtheria is a highly contagious bacterial infection that affects the throat and can lead to severe complications if left untreated. Whooping cough is caused by bacteria and affects the respiratory system, causing severe coughing spells and difficulty breathing. Tetanus, also known as lockjaw, is caused by a bacterial toxin that affects the muscles and can lead to stiffness and paralysis.

The DPT vaccine contains weakened or inactivated parts of these bacteria to stimulate the body`s immune system and produce an immune response. This helps the body recognize and fight off the actual diseases if exposure to them occurs in the future.

Therefore, option 1 is the correct answer as it correctly identifies the three diseases that the DPT vaccine protects against: diphtheria, whooping cough, and tetanus.