In which part of our body are Red Blood Cells (RBCs) formed?

In which part of our body are formed Red Blood Cells?

Antibodies are formed in blood plasma by which of the following -

In which organ of the human body are the lymphocytes formed?

The enzyme which takes part in changing fibrinogen to fibrin when blood clots are formed-

What is the main function of Red Blood Cells (RBC) in human blood?