The most important use of studying nuclear fission and fusion process was to harness nuclear energy for useful purpose of humanity. Nuclear reactor is the machine designed to harness nuclear energy to electricity. The basic principle involved is fission of uranium to krypton and barium by slow moving neutrons as in following ""_(0)n^(1)+""_(92)^(235)Uto""_(32)^(96)Kr+""_(56)^(141)Ba+3_(0)n^(1)+Q(200MeV) reaction: But the difficulty is that one neutron striking with ""_(92)^(235)U produces three(2.6 on an average) neutrons which can cause further fission The neutron intensity available for further fission is controlled by measuring reproduction factor defined as the average number of neutrons from each fission the causes further fission. Maximum value of K 2.6. For optimum operation of nuclear reactor, the value of Kis mentioned in Column-1 for various stages in Columu-Il match the correct columns : {:("Column-I","Column-II"),((A)K=1,(p)"Uncontrolled chain reaction"),((B)Klt1,(q)"Critical"),((C)Kge1,(r)"Sub critical"),((D)K~~25,(s)"Super critical"):}