Final answer:
Nerve cells are the least radio sensitive among the options provided because they do not reproduce as rapidly as other cell types, such as lymphocytes, making them more resistant to radiation. The correct option is 4).
Step-by-step explanation:
The question asks which of the following kinds of cells are the least radio sensitive: muscle cells, erythrocytes, lymphocytes, and nerve cells. Cells that are rapidly reproducing, such as lymphocytes and other types of blood cells found in the bone marrow, are typically more sensitive to radiation.
Muscle cells and nerve cells do not divide as quickly, making them less sensitive to radiation. Among the options given, nerve cells are the least radio sensitive because they rarely undergo division once they have maturely developed. Option 4) is the correct one.