The correct answer is option 2: white blood cells. White blood cells, also known as leukocytes, are responsible for carrying out the function of body defense. They are a key part of the immune system and are involved in protecting the body against various harmful substances, such as bacteria, viruses, and foreign particles.

Platelets (option 1) are responsible for blood clotting and are not directly involved in the body`s defense mechanism. Haemoglobin (option 3) is a protein found in red blood cells that is responsible for carrying oxygen throughout the body. While hemoglobin plays a crucial role in oxygen transport, it is not directly involved in the body`s defense mechanism. Red blood cells (option 4) primarily carry oxygen and nutrients to the body`s tissues and do not have a direct role in defense.

Therefore, the correct answer is option 2, white blood cells, as they are actively involved in defending the body against pathogens and foreign substances.