Which of the following is most susceptible to ionizing radiation?: (A) Mature bone, (B) Small lymphocytes, (C) Nerve cells, (D) Epithelial cells
Answer is B. Tissues that have high sensitivity are lymphoid organs, bone marrow, testes, intestines, mucous membranes. 
Tissues with intermediate sensitivity are ﬁne vasculature, growing cartilage & cartilage,  salivary glands, lungs, kidneys and liver. 
Tissues with low sensitivity include optic lens, mature erythrocytes, muscle cells and neurons.