Haemoglobin is an important component of red blood cells. Red blood cells, also known as erythrocytes, are responsible for carrying oxygen to different parts of the body. Haemoglobin is a protein found in red blood cells that binds to oxygen molecules and transports them from the lungs to tissues throughout the body. It also helps in the transport of carbon dioxide, a waste product, from tissues back to the lungs to be exhaled. White blood cells, also known as leukocytes, are responsible for fighting infections and foreign substances in the body. Plasma is the liquid component of blood that carries various substances such as nutrients, hormones, and waste products. While haemoglobin is important for the functioning of red blood cells, it is not a component of white blood cells or plasma. Option 2, red blood cells, is therefore the correct answer.