Which cells within the immune system identify and destroy foreign bodies such as viruses?

Leucocytes

The correct answer is Option 4: Leucocytes

Leucocytes (white blood cells) are the main defensive cells of the body. They include different types like T cells, B cells, and natural killer cells, all of which work together to recognize and eliminate pathogens like viruses. Since the question asks for the broader category of cells responsible for defense, leucocytes is the most appropriate answer.

NCERT: "Leucocytes, also known as white blood cells, are the cells within the immune system that identify and destroy foreign bodies like viruses. It also leads to the production of antibodies. There are several kinds of white blood cells or leucocytes within the immune system, including T cells, B cells and natural killer cells. T cells destroy invaders, and T-helper cells increase immunological activity. It is these T-helper cells that are attacked by the Human Immuno Deficiency Virus (HIV), the virus causing Acquired Immuno Deficiency Syndrome (AIDS). B cells produce antibodies. Natural killer cells are involved in the fight against both viruses and tumours."

The other 3 given options are types of white blood cells.