Why A? And why not the others?

Correct answer: A. In the red bone marrow

In adults, red blood cells are made in the red bone marrow, chiefly inside the vertebrae, ribs, breastbone, and pelvis, where precursor cells fill with hemoglobin, eject their nucleus, and slip into the bloodstream. The scale is staggering: the marrow releases millions of new cells every second just to keep pace with losses. The spleen is a brilliant trap answer because it is where worn-out red cells are destroyed, the opposite job, and it does make blood cells before birth. The liver is tempting for the same reason, since it is the main blood-cell factory in the fetus, while the heart only pumps blood and never manufactures it.

Source: Encyclopaedia Britannica: erythropoiesis