Solution

Hemophilia is X-linked recessive disorder and is characterised by impaired ability or inability of blood clotting which causes heavy blood loss even in the case of small injury.
Pacemaker serves to produce rhythmic impulses for a heartbeat. The sinoatrial node (SA) node serves as a pacemaker.
Malleus is one of the ear ossicles. It is also known as a hammer.
Leucocytes refer to white blood cells of blood which serve in non-specific immune response. Lymphocytes are the immune cells and serve in specific immune response. Correct match is B.