Leucocytes are also known as:

Correct option is B

Sol. Leucocytes are white blood cells (WBCs) responsible for protecting the body against infection. They are part of the immune system and are found in blood and lymphatic tissue.
Explanation of each option: • (a) Red blood cells (RBCs) – Carry oxygen through hemoglobin but are not part of the immune system. • (b) White blood cells (WBCs) – Correct. These fight infections and include various types like neutrophils, lymphocytes, and monocytes. • (c) Plasma – Fluid portion of blood that carries cells and nutrients but is not a cell itself. • (d) Haemoglobin – A protein in RBCs that transports oxygen; not related to immune function.